Those big blue connectors should not be used outside the body of the trailer, better yet not used at all. The other butt connectors should be the ones that shrink to the wire when heated, and I have some that have solder built in them that melts when heated for a good connection. Uhauls are kind of odd in that each side is wired separately, that is, one of those black cords feeds the left
tail lights and some of the marker lights that are tapped in below the sink, and the other cord feeds the right
tail lights and the rest of the markers that tap in just to the rear of the
furnace. I rewired ours with the flat four wire harness. Ran it along the outside of the tongue, up through the floor near the
furnace where the original wire went in, back to the right lights and connected the right wiring for the markers there. Then across the rear inside over to the left lights, and then on to the marker wiring below the sink. Eliminated all that old frayed cracked original wiring. I didn't try to feed the new wiring through the tongue, because it could wiggle around in there and eventually wear through and short out. The less connections out in the weather, the better. Here in the Northeast, wiring on tractor trailers was impossible to make a permanent fix, no matter what we did salt would eventually work in and cause corrosion. I'm sure lights and wiring was what we spent most of our time on. I'm retired from that work over a year and a half now, and don't miss it at all. Bob