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Installing the Main Trailer Wiring Harness

Wires under a utility trailer splitting off laterally from the main beam behind the coupler

For lack of a better term, what I mean by the "main harness" is the front part of the trailer wiring harness that extends from the vehicle connector up to and including the wires that branch left and right along the transverse beam under my particular trailer.

As you can see in the picture, all the wires come through the forward beam that goes to the coupler, then branch out laterally into steel tubes welded onto the frame. From there, they go to the marker lights, and then aft to the rear lights through two more metal tubes mounted topside.

My trailer is rather unusual in this regard in that it has only one main beam at the leading edge, so the wiring splits at a transverse beam under the trailer in an inverted T shape. Most trailers have two main supports in the front in an inverted V or Y shape, so the wiring splits at the coupler and runs down each side.

From a practical perspective, all that meant was that when replacing the trailer wiring, I had to spend some time under the trailer to run the lateral wires, with the front of the trailer lifted up on jack stands and the wheels chocked. That was okay. I also found some stuff I'd thought I'd lost while I was under there.

The Procedure

Running the harness wasn't exactly rocket surgery, nor even brain science. This is the basic procedure I followed.

  1. After I sheathed the harness, I tied a piece of string to the front eye of a fishing pole and pushed it through the main beam from under the coupler.
  2. I tied that string to the harness and carefully pulled it through the main beam to the transverse beam from under the trailer.
  3. The next step was to slip the grommets over the wires before running the wires through the metal tubes attached to the transverse beam, into which said grommets would have to be inserted.
  4. I ran the wires to the left marker and rear lights left, and the wires to the right marker and rear lights right, through the tubes welded to the transverse beams.
  5. I clipped the wires to the beam using the same clips the manufacturer used, rust and all. I'll replace them with shiny new trailer wire clips the next time I place an order with Amazon.

And that was basically that. Not much to it. But here's a video anyway.

Video: Installing the Main Harness

The next step in my trailer-rewiring project was to connect the ground wire and hook up the marker lights.