Trailer Wiring Harness-Custom guides you need to know

Does your trailer have the right wiring system? With time, the wiring system in your trailer may need a relook. Are you worried? Don’t be. It’s time to design your trailer wiring harness, one that will fit your trailer and your vehicle. With the right guide and a little bit of help, you can ensure that it’s simple to wire your trailer the right way.

In this guide, you will learn about the common wiring systems, the different ways to wire your vehicle for towing, and how to convert different wiring systems. You will also get to know how to rewire your trailer using a trailer wiring harness.

Get Your Free Sample!

Enjoy a free sample worth UP TO $100!
Explore our custom services now. Email us at [email protected] for more details.

Chapter 1: Types of Trailer Wiring Systems

People attach all kinds of trailers to their vehicles. You can attach a boat trailer or towing a trailer to your car to tow on the road. To use the trailers on the road, though, you have to make sure they abide by laws and regulations. One such law is having working lights and turn signals that sync with your vehicle. That means you got to have working tail lights, brake lights, and turn signals. You also need them to make way for safe towing and alerting other drivers on the road.

Before you wire your trailer, you need to know the wiring system of your vehicle. We will discuss some common vehicle wiring systems used by cars and automobiles.

Two-Wire System

Two-wire systems are the most basic type of wiring found in vehicles and trailers. The system uses one wire to transmit the stop or brake signals. The taillight signal flows through another wire. You can find two-wire systems only in some vehicles right now.

Three-Wire System

As the name suggests, three-wire systems come with three wires. It uses three different cables to transmit the stop, turn, and tail signals. You will mostly need a converter to connect three-wire systems to your trailer. It’s also the most common system used in the automobile industry.

Pulse Width Modulation (PWM) System

Pulse Width Modulation (PWM) System is also known as a multiplex system. The system uses a single wire to send stop, turn, and taillight signals. It does so by using signals of varying intensity. Most of the current vehicles use this wiring technology.

You can use LED with PWM systems which fall into two categories-

ST systems: ST system refers to the stop and tail system. It takes the help of a single wire to transmit taillight and stop signals. You have separate wires to send the turn signals.

You may need an ST converter to connect the vehicle wiring to the trailer wiring.
STT systems: STT system refers to stop, turn, and taillight signals. The system uses a single wire to control all three- stop, turn, and tail signals.

You will generally need an STT converter to hook up your vehicle wiring to trailer wiring.

Wiring system of modern vehicle taillights

Now, we will show you how to prepare your vehicle for towing.

Chapter 2: How to Wire Your Vehicle for Towing

You will need something to connect your vehicle wiring to trailer wiring. Some vehicles come with a built-in connector to facilitate connection. If your car doesn’t have a connector, then custom wiring is the ideal answer.

You can customize custom harnesses to match the wiring of your vehicle. A custom trailer wiring harness is also known as T-connector. It provides a socket to connect to the trailer, commonly using a 4-way flat.

A custom harness may or may not need an integrated converter. You will only have to use a converter when your vehicle requires it.

Types of Custom Wiring

You can use two types of custom wiring for connecting your vehicle wiring to the trailer-

  • Custom wiring harness
  • Custom wiring connectors

Both may seem identical look-wise, but they differ entirely in function.

Custom Wiring Harness

A custom wiring harness comes with several plugs. The plugs can connect directly to the taillight assembly of the vehicle. The harness sources its power from a battery or the taillights. You will need two connection points to set up a connection using a custom wiring harness. You don’t generally need any splicing or soldering.

Custom Wiring Connectors

Not all vehicles have a connector to hook up trailer wiring. They may come with a special socket provided by the manufacturer. The socket helps you to connect your vehicle wiring to trailer wiring. The system uses a single plug to connect to the factory-made socket of the vehicle. It helps you set up the connection using standard trailer wiring connectors.

You can get custom wiring harnesses for your trailer

Check out whether your vehicle has a socket for connecting trailer wiring. Then decide if you need a custom wiring harness or a custom wiring connector.

In the next section, we will explore the ways to convert two different wiring systems.

Chapter 3: How to Convert Two Different Wiring Systems

You may not always get a custom wiring harness for your trailer. In that case, you will need an electrical converter. An electric converter helps you connect your vehicle’s wiring with the trailer. It provides a way to hook up complex vehicle wiring with simple trailer wiring.

For instance, most cars use the three-wire or PWM system. On the other hand, trailers generally use two-wire setups. Now, the two configurations do not match. Here, an electric converter comes in handy. It allows you to connect the vehicle and trailer through a regular plug and socket.

You can try searching your local automobile spare parts shop for electric converters. Else, you can also search online.

Use electrical adapters to convert different wiring systems

Now comes the turn to find out the differences between plugs and sockets.

Get Your Free Sample!

Enjoy a free sample worth UP TO $100!
Explore our custom services now. Email us at [email protected] for more details.

Chapter 4: Differences Between Plugs & Sockets

Plugs and sockets help you connect the wiring of your vehicle with the trailer. Socket refers to the connection that resides with the car. Plug refers to the connection of the trailer side. You may find plugs and sockets with two to seven wires. The electrical requirements of the trailer determine the number of wires you need.

Most plugs and sockets have color-coded wires to facilitate easy connection.

Common Plug Ends by Trailer Type

Trailers come with different types of plugs. The electrical components of the trailer determine the type of the plug. Here is a list of different trailers and the plugs they use-

Boat trailer without surge breaks: 4-way flat plugs are most common. You may also find 7-way RV blade plugs in some cases.

Boat trailer with surge breaks: Commonly uses 5-way flat plugs. On rare occasions, they may also use 6-way round or 7-way RV blade plugs.

Boat trailers generally use 4-way flat plugs or 5-way flat plugs

Utility trailers: 

4-way flat plugs are most common. Sometimes they also use 6-way round or 7-way RV blade plugs.

Camper trailers: 

Camper trailers generally have 7-way RV blade plugs. At times they may also use 6-way round or 7-way round plugs.

5th wheel trailers: 

 uses 7-way RV blade plugs. Few of them may have 6-way round plugs.

Gooseneck trailers: 

6-way round plugs are most common. Rarely do they use 7-way RV blades plugs.

In the next part, we will learn how to adapt to two different connector types.

Chapter 5: How to Adapt Two Different Connector Types

You will find countless types of connectors in vehicles. That means your trailer connector may differ from that of your car. For instance, your vehicle may have 6-way round connectors while the trailer uses a 4-way flat. How do you establish a connection in such cases?

The answer lies in using a simple adapter.

An adapter is a simple electrical device. You can use it to connect your vehicle wiring to trailer wiring in case of different plugs and sockets. Adapters don’t need any extra work to establish the connection. They act as a bridge between two different connector types.

In rare cases, you may need to use extra wires to ground the system. You may also need wires to splice into your current vehicle wiring. Some common types of adapters include-

  • 4-way flat to 6-way round adapter
  • 4-way round to 4-way flat adapter
  •  7-way RV blade to 6-way round adapter
  • 7-way RV blade to 6-way round adapter
  • 6-way round to 4-way flat adapter
You can use adapters to match different connector types

Now we will tell you how to rewire your trailer to make it road-ready.

Chapter 6: How to Rewire Your Trailer

Rewiring your trailer is not a difficult task. You will only need a few tools and some of your time to do the job. The main thing to arrange is your trailer wiring harness.
Tools You Will Need

1. Trailer wiring harness

2. Pliers

3. Wire brush

4. Clips

Follow the steps mentioned below to rewire your trailer. If you are not confident enough, you can also go to a professional for the task.

Set Up Your Trailer

Step 1: Take Out Old Wiring

Remove the old wiring of the trailer using a set of pliers. Also, remove the trailer wire connector if present. Keep the layout of the wire in mind to assist you in rewiring the trailer.

Step 2: Clean the Area

Use a wire brush to clean any corroded material from the space of the old harness. You can also spray the area with anti-rust paint to check corrosion.

Step 3: Re-Route the New Wiring

Now place the new harness through the frame to reach the back of the trailer. Try to route the wire along the frame rail of the trailer. You can use rubber grommets to secure the harness.

Step 4: Connect the Ground Wire

Check if your trailer wiring harness came with an attached plug. It would help if you connected the white ground wire to the trailer connector. Then you don’t need to do any wiring at the coupler.

Be sure to leave enough wire at the connector to reach the trailer plug. Also, allow enough slack in the wire to accommodate the turning radius of the trailer.

Installing the Lights

Step 1: Mount the Marker Lights

Use the bolts to install the marker lights. Then use snap locks to splice the wires of the marker lights.

Step 2: Secure Wires to the Frame

Use the clips from the wiring kit to secure the wiring to the frame. Protect the harness by the trailer frame to keep it safe from contaminants and movable parts.

Step 3: Mount the Taillights

Now attach the taillights to the frame of the trailer. Then connect the wires to respective connection points.

Connect the wires to your trailer taillight

Test Your Connection

It’s always better to test your trailer lights before you hit the road. Also, make sure that the lights work in sync with your vehicle’s lights.

Pro tip: Most electrical issues arise from problematic ground connections. It may result in lighting issues even when your voltmeter displays the available charge. For best results, use the manufacturer-provided factory ground connection. Else, connect the ground wire to the chassis using a bolt.

In the next section, we will tell you where you can get a custom trailer wiring harness.

Chapter 7: Where to Buy Custom Trailer Wiring Harness?

Custom wiring trailer harnesses may be available from multiple sources. Here are some of the avenues you can try to buy your custom harness.

Local Automobile Spare Shop

You can check out your local automobile spare shop for a custom harness. They may have a harness in stock that suits the needs of your vehicle. Be sure to note the specifications of your vehicle and trailer wiring. That will help you select the right harness for your car and trailer.

You can find your trailer wiring harness in your local automobile spare shop

Online Stores

Many online electrical stores deal in automobile harnesses. You can search the internet and browse various online stores to locate your harness. Make sure to determine the reliability of the seller before placing your order. Also, look for fair return policies.  You may need to return the harness if it doesn’t match your requirements.

Cloom Tech Supply Custom Trailer Wiring Harness

You can also get in touch with us to create your custom trailer wiring harness. We make custom harnesses that match the specifications of your vehicle and trailer. We adopt strict quality control procedures and meet all professional standards. If you are looking for harnesses tailored to your needs, we have got you covered!


Trailer wiring harnesses are crucial if you want to tow a trailer. It helps you connect your vehicle wiring to trailer wiring. Without a harness, your trailer lights and turn signals won’t work. That means you can respect the law and avoid getting tickets using a harness. You can contact us for high-quality custom trailer wiring harnesses. We manufacture custom harnesses with high standards matching your exact specifications.

Get Your Free Sample!

Enjoy a free sample worth UP TO $100!
Explore our custom services now. Email us at [email protected] for more details.

Hey, I am John, General manager of Cloom and OurPCB.

I am a responsible, intelligent and experienced business professional with an extensive background in the electronics industry.

Reach me at [email protected] to get a quote for your projects.


Fill in your details and we’ll get back to you in 24 Hours.

contact us


Fill in your details and we’ll get back to you in 24 Hours.