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Installing the Side Wires and Rear Trailer Lights

The author using his hands and a crimping tool to connect the rear lights on his trailer when replacing its wiring

The final step of my DIY trailer-rewiring project, after installing the ground wire and the marker lights, was to install the braided sleeving on the wires that run down the sides of the trailer to power the rear lights, run them through the tubes on the sides of the trailer, and connect the rear lights.

On a trailer that uses a four-wire trailer wiring harness, the brown wire powers the marker lights and tail lights on both sides. The yellow wire operates the brake lights, turn signals, and hazard flasher on the left; and the green wire operates those same functions on the right.

My trailer uses a Y-harness runs down the center beam from the coupler to a transverse beam under the trailer, then splits left and right in an inverted T manner. From there the wires go to the marker lights on each side, and then run to the back of the trailer through metal tubes mounted on each side.

On some trailers, however, the harness runs down one side or the other or down the middle, and wires have to be run laterally to the lights. I mention that only because I assume some people will wind up on this site because they're rewiring their own trailers, and I wanted to mention that not all trailers route the wires the same way.

The Procedure

  1. I ran the brown and yellow wires down the left side of the trailer and the brown and green wires down the right side through the metal tubes on each side of the trailer.
  2. I stripped the ends of the wires I just ran and the wires coming from the lights using an automatic wire stripper.
  3. Then I connected the wires to the corresponding color-coded leads of the rear lights using heat-shrink butt splice connectors, a Wirefy crimper, and a heat gun.
  4. I covered the connections with heat-shrink tubing for structural rigidity and protection from the weather.
  5. Finally, I tested the lights.

When testing the trailer lighting, you should check the tail lights, the brake lights, the turn signals, and the hazard flashers. Even though the stop lights, turn signals, and hazard flashers on each side all work off the same wire, you also want to assure that the trailer lighting converter on the tow vehicle is wired correctly and working properly.

One thing I should mention because it's not clear in the video is that the threaded posts coming out from the rear lights and through the trailer frame are there for mounting and grounding purposes only. Do not connect any wire other than a ground wire, if needed, to those posts. You will cause a short-circuit if you connect any other wires to those posts.

That about sums up this last step of my trailer-rewiring project. For the long story, check out the video below.

Video: Installing the Side Wires and Rear Lights on My Utility Trailer

This was the final step of my utility trailer rewiring project. I hope you enjoyed the site.


A utility trailer tail light. The side marker light of a utility trailer. A tray full of heat shrink wire connectors of assorted types and sizes. A crimping tool being used to crimp an electrical connector on a utility trailer. A few inches of heat shrink tubing over the wires of a utility trailer. A heat gun being used to shrink and attach an electrical connector to a wire. A wire stripper being used to strip the ends of the wires being installed on a utility trailer. Wire loom installed over the wiring of a utility trailer to protect it from damage. A floor jack being used to lift a car.

The gray-bearded author outdoors with a wild bird on his shoulder and a Buy Me a Coffee tip link