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How To Wire A Trailer

If the lights have gone out on your trailer, it is time to redo the wiring. There is no sense attempting to patch or repair issues with faulty wiring systems when replacement is a quick and easy option. Wiring trailer lights is not as difficult as most anticipate. This project requires a trailer lighting kit that includes a wiring harness, small connection clips, a couple rear turn signals and a couple side mounted lights.

Trailer Light Wiring Basics

It will help to have an understanding of trailer light systems and we recommend reviewing trailer wiring diagrams before starting this project. Though trailer lighting systems are available in an array of configurations, the most popular is the 4-way system. It is referred to as such as the system needs a mere four wires for the trailer tail lights, turn signals and brake lights to illuminate. The 4-way system is designed so the right brake light wire and right turn signal are green. The left brake wire and left turn signal are yellow. Side marker lights are powered by the brown wire. White remains as the ground. This is the general configuration upon which 5-way, 6-way and 7-way wiring systems build upon. As an example, you will find another blue wire in a 5-way system that powers auxiliaries like the electric brakes. If your vehicle has a 6 or 7-way connector and you have a trailer system that is a 4 or 5-way, pick up an adapter so you don't have to bother splicing in a new connector.

Removing the Old Lights

This is usually the most challenging step of trailing wiring projects. The connecting nuts and bolts of the old assembly are likely significantly corroded. Your light assembly kit will have the new hardware you need so go ahead and remove the aged lights even if it requires significant force. You might have to use a bolt cutter or pry bar to get the job done. Don eye protection to safeguard your eyes during the removal of the old lights.

old lights

Where to Start

Begin by using a circuit tester to ensure each wire offers the necessary power. Verify the white wire is firmly connected to the frame. Check for conductivity between the frame and the trailer connector. The trailer's metal frame almost always functions as the lighting system's ground. If you have a poor ground, all sorts of problems can arise like the brakes illuminating in an uneven fashion. So check the boat-vehicle connector to ensure the white wire is firmly attached and void of corrosion. If it is dirty, corroded or faulty in another manner, clean it and reconnect it.

Use one of the connectors provided in the kit to hook up the new and old wire. Pull out the old wire while pulling the new wire on through. The next step is to connect the new lights. Remove the front cover to attach the wires. Attach a grinder to your brush to eliminate any corrosion on the plate that connects to the light. The middle light will require good ground for functionality so you will have to get right on down to the bare metal. Attach the light, pull the small black wire near the dual wires that go toward the rear light. Grab the little clips included in the kit to connect the wire moving from the side light to the central wire that moves toward the rear lights. Place the wire you want to send power to in the clip and crimp down on the metal tab. Fold the small piece of plastic atop the entire assembly to form an aesthetically pleasing connection. Repeat this process on the other side for full illumination of your trailer.

where to start

Points of Note

Be sure to clean the section where the ground is applied. Use sand paper or a wire brush to clean off this area. It will also help to apply a coating of water-resistant caulk or a spray-on corrosion inhibitor to the scotch locks used to secure the wires. This coating will protect against corrosion and moisture. If you think you can get your lights up and running without replacing the complete setup we recommend reading trouble shooting trailer lights.


4-PIN - This is one of the better videos we have come across that shows how to wire the tail lights on a utility trailer with a 4-way trailer wire connection.

7-PIN - This is an excellent in depth video on how to wire a 7 way round trailer plug that includes wiring diagrams.

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