Finally bringing Prostar home to fix....

[Chamberpains]

Check voltages in every wire you can. And ohm out every ground you can while you have it apart. It literally can be any curcuit cross feeding in to another from almost anywhere on the truck. Almost all 12v truck circuits will run on either 12v or 5v. So if all 5v curcuits are showing 4.2v or whatever and you find one that is at 2.1v then that would be a suspect curcuit to run the wiring out on. Same with your 12v curcuits and ohming out the direct grounds. The odd ball out is usually gonna run you into a problem child. I say A problem child cause I always found more than I bargained for. But needed to be fixed none the less.
I also say direct grounds because if your ohming out a ground that goes to a resistor or resistor switch like in the heater controls than the ohms are gonna be anywhere.
I've had many truck apart looking just like yours. Its a hair pulling mess that used to enjoy. But once your that deep physically cutting tape and zip ties and pulling kick panels and wall panel to see wires has yielded good results for me on those week long dismantling projects. I found taking shots in the dark almost always returned nothing and cost more time and money and lead me right back to getting hands on with everything. It can always be found or at very least be isolated and wired around. Sometimes i would also document wire numbers and plug formations i verified good on a piece of papper so when i needed a break or it bled into days, i wouldn't redo stuff i already verified.