[JRTJH]

I'd again suggest "jumpering" the umbilical as described above and shown in a photo from a kind poster. You need to "RULE OUT THE TRAILER" or "DEMONSTRATE THE TRAILER FAULT". Once you know IF the trailer works or doesn't then you can move to the tow vehicle. Right now, you're not sure if it's the truck or maybe the trailer, there's 14 VDC, but not in some places, and unsure where the voltage drops.

If you jumper the umbilical and the lights work, look at your truck as the problem. If you jumper that wire and the lights don't work, it's in the trailer, not the truck. Then you can focus on which end of the umbilical connector to start working on. Right now, you're working on both ends against the middle without any defined problem.....