I completely rewired our Uhaul
. I found that some of the original wiring was dry rotted and split open under the trailer exposing bare wire. The way your trailer is wired is you have those two wire cords that go into the tongue. They come out the tongue under the trailer. One cord goes to the right side (curb or door side) and up through the floor about under the closet/furnace area. The other cord that comes out of the tongue under the trailer goes up through the floor under the sink. Left cord goes to left tail light
and some of the marker lights, right cord goes to right tail light
and the rest of the marker lights. I don't remember what marker lights were connected to each cord, doesn't really matter. I believe it is wired this way with the two cords so that when the trailers were rented they would tap into each tail light
on the tow vehicle. Uhaul
had replacement bulbs that had wires coming off them for this purpose. I have a set of them. What I did with my rewire job is this: Ran new 4 wire flat trailer wire along the tongue not in it, up through the floor on the right side, back to the right tail light. I am only using the red lens part of the tail light. Made my connections at that light, ran wire from the right light, across the rear inside of the trailer over to the left light, made my connections at that light. The marker light wires come down from between the walls, on the left & right sides, about where the original cords came up through the floor. I spliced into the tail light and ground wires at each tail light with a two wire jacketed wire harness and ran that forward to tap into the existing marker wires that hang down between the inner & outer walls. If you want to also use the amber part of the tail lights
there is a converter that will allow you to do that. I'm assuming in all this that your tow vehicle has the 4 wire flat plug?? I have a rear mounted bike rack, so I put an extra set of stop/turn/tail lights on it that is powered by a completely separate wire harness that is tapped into the plug at the hitch.