How to hook up trailer wire harnesses?

You must attach trailer lights to your vehicle if you’re towing a trailer for public safety.

While every state in the U.S. has laws and regulations governing everything of the tailor lights, from size to weight restrictions, they all have one thing in common.

They must be wired for taillights, brake lights, and turn signals.

For that, you can connect the trailer to the electrical set-up of the vehicle.

However, you will find an unending variety of cables with sheathing of different colors and connectors.

What’s that? And how do hook up trailer wire harnesses?

Trailer wiring diagrams can help.

How do you attach the trailer lights?

Using T-One connectors is your best solution.  

T-connectors, also known as custom wire harnesses, are designed to work with a specific vehicle while providing simple output, with no additional wiring or splicing necessary.

4-way flat trailer harnesses are commonly used.

There are also 5-pin, 6-pin, and 7-pin trailer harnesses available in flat, round, and square. 

It has multiple plugs that ‘T’ into the vehicle’s taillight assembly so you can draw power directly from the taillights or the battery. 

Taillights, turn signals, and brake lights are essential for any trailer.

Markers and flashing lights are also necessary for some roads.

In addition, electricity is needed for some brakes, such as when you want to reverse and engage the electrical brakes or turn off the hydraulic brakes.

No matter your condition,  T-One connectors are the most convenient option because they have an OEM-style link that fits into the car’s current wire harness. 

Why is the color code for trailer wiring important

Every wire in your trailer wiring has a purpose; one for the tail light, one for the left brake light,  one for the right, and so on.

Following the color code is important because mismatched wires can confuse the road if the trailer lights aren’t working properly.

Connector StylePINFunction Color Description
1 GroundWhite Provides ground to all trailer connections
 Side Markers Tail and Runner Lights
3Left Brake, Light, and SignalYellowMulti-function signal for the Left Side Rear Tail Lamp
4 Pin 4Right Brake, Light, and SignalGreen Multi-function signal for the Right Side Rear Tail Lamp
5 Pin Connector5BrakeBlueControl Power of Hydraulic Brake
6 pin Connector6BatteryBlack or Red12 V power
7 pin Connector7Back-upPurpleBackup Lamps and hydraulic breaks

When do you apply the 4-pin trailer connector?

A 4-pin trailer connector is common in most trucks since it is the industry standard for trailer wire harnesses.

Thus, utility trailers use this plug to keep the vehicle safe while towing.

Four wires are connected: green to the right or brake, yellow to the left turn or brake, brown to the taillight, and white to the ground. 

When do you use the 5-Pin Plug?

You might not see a 5-pin plugin in many trailers since it is less common than the 4-pin.

However, this added pin has a major significance in many cases.

The fifth wire is usually blue and helps connect the reverse lights.

Since not all trailers have reverse lights, you will not always need the 5-pin plug.

For example, in boat trailers, 5-pin plug wiring is necessary. 

When do you apply the 6-Pin Plug?

Many companies use the gooseneck trailer to deal with heavy-weight objects.

These are usually placed on the truck’s bed. For such situations, the 6-pin plug system works perfectly.

It has the same color scheme as the others, with an added feature of black or red auxiliary wire.

Also, most wires in 6-pin are round. 

How about the 7-Pin system?

RVs and other big loads typically require the 7-pin system.

You can find many kits to help you with 7-pin wiring in the market.

Their color and function may vary; hence check what the colors mean before connecting the system.

Also, our favorite is the 7-pin spade and 4-pin flat connectors, which provide round and flat pull from one location.  

Trailer taillights

Trailer taillights


Trailer wiring is not so complicated if you have the right kind of equipment and skill with you.

You only need to look for the schematic and wire it up in an actual way.

If trailer wiring isn’t your thing, let Cloom handle it. 

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.


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