Overland Trailer Electronics

DIY Overland Trailer Build Part 6: Wiring Harness and Tail Lights

After enclosing the cargo box for my DIY overland trailer, the next step of the build was installing the trailer wiring harness that connects to my vehicle’s electrical system and installing tail lights.

In California, where I live, and most other US states (if not all of them), trailers over a certain size must have brake lights and turn signals. This requires connecting the trailer to your vehicle’s electrical system so the lights on the trailer respond when you hit the brakes or indicate a turn.

4-Pin vs 7-pin Trailer Wiring Harness

There are two types of wire harnesses typically used for pulling overland trailers: 4-pin and 7-pin. A 4-pin harness is the most basic and will typically only provide power for the trailer’s lights, allowing for running lights, turn signals, and brake lights.

A 7-pin harness, on the other hand, will also provide power for the trailer’s brakes and auxiliary devices such as a second battery and associated electronics.

7-pin trailer harness
7-pin trailer wiring harness from Mictuning.

I opted to install a 7-pin wiring harness, as the Timbren hubs I used for the trailer’s wheels have electronic breaks. I also eventually plan to install a battery-centered electrical system, charged by solar panels and the Jeep’s engine. This will power lights, a portable refrigerator, and other amenities. If you don’t need those functionalities, a 4-pin harness will do the job.

Installing the Wiring Harness

The first step was to install the wiring harness that would connect the vehicle’s electrical system to the trailer. I opted to use a 7-pin harness from Mictuning. I also had to replace the 4-pin harness on my Jeep with a 7-pin wiring system specifically designed for Jeep Wranglers.

For this phase of the project, a few specialty tools and supplies were required.

Wire stripper
A tool for cutting, stripping and crimping wire is essential.

The primary tool I used was a wire stripper that could also cut wire and crimp connectors. This is an essential tool for working with electronics and is well worth the money.

I used a 14 gauge wire to connect the wiring harness bus to the electrical components. I bought a bundle that included wires of various colors so that I could match the colors of wire in the bus box. This made it easier to keep track of the different circuits.

14-gauge wire

To connect the wires from the front bus to the electrical components on the trailer, I used heat-shrink, butt-spice wire connectors. In places where I thought I might need to disconnect the wiring, I used quick disconnects.

You can use a lighter to activate the heat shrink tubing on the connectors. I have a MAPP gas torch, so that’s what I used, as it goes a bit faster (though you have to be careful not to burn the plastic).

The wiring harness for the trailer is pretty simple. There’s a circular connector that attaches to the 7-pine connector from the vehicle, a long cable, and a wiring bus that serves as a junction for the wires that connect to the electrical components on the trailer.

7-pin connector from trailer wiring harness.

The wiring harness comes with a guide that tells you what the different colors of wires on the bus correspond to — eg, brakes, turn signals, brake lights, auxiliary power, etc.

Diagram for 7-pin Trailer Wiring Harness
The 7-pin wiring harness came with a diagram that showed which color wire corresponds to which functionality on the trailer.

To install the wiring harness, I inserted rivet nuts on the frame, near the base of the trailer tongue. These receive the bolts to hold the wiring bus box to the frame. I added another rivet nut to the frame to attach the wiring harness ground wire to the frame. This allows you to ground other electric circuits anywhere on the frame, so you don’t have to run a ground wire all the way back to the wiring bus box for each circuit.

Trailer wiring bus box
The wiring harness bus box is attached to the trailer frame. The wire coming out of the left side goes to the Jeep. The wiring loom to the right (with the ridges) carries wires to the trailer’s electrical components.

In retrospect, I should have located the box in a different spot, as I plan to put a storage box on the tongue area that will need to be attached to that location. I’ll need to move it when I get the box.

Trailer wiring diagram
Hand-drawn trailer wiring diagram.

To plan for connecting all of the electrical components on the trailer to the bus on the 7-pin trailer wiring harness, I drew a diagram. This was really helpful to keep track of what I was doing.

Mounting the Tail Lights

There are several styles of tail lights for trailers on the market. I originally planned to mount the tail lights on or above the fenders of the trailer, with the idea that this would be more visible and protected.

After thinking it over for a while, I decided this would require me to run wiring longer distances than necessary. Instead, I opted to mount the lights on the trailer’s bumper.

Trailer lights

I bought a set of long, thin LED lights that serve as brake, turn, and running lights. These lights have a hard plastic nub in the middle of their back where the wires for the lights emerge.

Trailer lights
Each light required three holes in the bumper: one in the middle for the wiring and two on the sides for nutserts to mount them.

I drilled a hole in the center of the front side of the bumper so that wires from both of the tail lights could come out of the bumper at the same spot. This allowed me to run wires from the 7-pin bus at the front of the trailer to a single location on the bumper to wire up the lights.

Tail light wires pulled through a single hole in the front side of the trailer.

Often, people will run the wires from the trailer wiring bus through the frame to connect with the lights, brakes, and other components. I may do this eventually, but I was reluctant to drill additional holes in the frame, which could lead to corrosion. Also, I might want to change how the trailer is wired as I use it more, so I wasn’t ready to commit quite yet.

Instead of going through the frame, I ran the wires along one side of the trailer and protected it with a plastic wiring loom. I held it in place with stick-on brackets and zip ties. It’s probably not a permanent solution but will suffice for now.

One thing that will make your life easier (and save you money) if you are wiring a trailer, is to ground the various electrical components to the frame, instead of running a ground wire all the way back to the wiring bus box at the front of the trailer. If you have grounded the bus to the trailer, you can simply attach the components to the frame wherever they happen to be located.

It took me a few times to get the wiring worked out for the lights, as they didn’t come with clear instructions on what wires did what. Eventually, I figured it out and got the brake lights, running lights, and turn signals to all work properly with input from the Jeep. Very satisfying to get it all wired up.

In this stage, I only connected the tail lights and the brakes. In the future, I plan to add a battery system to power electrical components, which I’ll document when the time comes.

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