2011 Pete 386 Tail Lts

I've got a 2011 Pete 386 ISX Cummins, 18spd. Whenever I plug the power into the trailer (any trailer we have tried 4 different ones) It knocks out the dash lights and the clearance lights on front and back of semi and whichever trailer I hook up to. Traced many ires found a few corroded and fixed them. Also, when it started having this issue the ground from the trailer pigtail to truck female receptacle got hot and partially melted wires where the truck female receptacle wires are attatched via the small Philips screws on the backside of the truck receptacle. Fixed that, followed wires to firewall and there doesn't seem to be any issues there. Inside I have battery power at the fuse (clearance lights) but the main wire at the firewall G4 only shows 8 volts. Approx a 4volt drop between the fuse and that particular wire. When the clearance lights switch is on the brown wire on the pigtail also shows 8 volts. Black on pigtail shows battery voltage. Could this be the computer? Pulling my hair out checking wires. The other issue is this happens most of the time hooked to a trailer but not all the time and intermittently without the trailer. Any ideas greatly appreciated...