1988 Peterbilt 379 Tail lights and side markers.

Best course is to put back as original or as close as possible. Sounds like they just put wires to other switches as some went bad. Strip those wires back on bad connections to good wire and use heat shrink connectors as a minimum. If the wires for the switches are are hard and the insulation is burnt cut them back and add wire if needed.
If you post the serial number and ask someone maybe able to send you a wiring diagram. With that a savy guy can put it back correct. Good luck and keep the pics up.

 

Super Miller had most of the useful wiring diagrams for figuring that stuff out. If not shoot me an IM and I’ll email what I have.

Mine’s an ‘89 and I’m in the middle of a bunch of rewiring. Fixing work done by morons that couldn’t/wouldn’t figure out what the actual problem was so they cut wires and ran new ones.

 

Be careful with that fiber optic material. If you'r lucky, when you get it put back together, all the labels will light up.

 

That could be a really nice "free" truck with some TLC and a lot of $$$.
I would replace the entire rear wiring harness, clear up to wherever it connects under the hood, and maybe the dash harness too.
I know there are companies that can rebuild wiring harnesses, if Peterbilt doesn't sell replacements.
Usually found under classic car type websites.

 

I know this is a old post hopefully I can get a response. I have a 90 model 379 the wiring isnt in to bad on it all my lights and switches work except my brake lights. I have running lights and turn signals but I cant get my brake light working. The relay for the stop lights is one of those old style white buttons that as I know isnt replaceable. Any suggestions on how to get them working? Thanks

 

The brake light switch is behind the panel by the steering column in 1 of the air manifolds. Probably bad and need to replace.

 

The thing with the white button is probably a circuit breaker. Does it have power on both terminals? If you unplug the turn signal switch harness do you have power on the truck harness when you mash the brake? Then you know which way to chase. Must have air and possibly key on when testing.

 

I would start with checking the air switch that @mslashbar spoke of. You can check the continuity of the switch with/without braking application to see if the switch is good before replacing it.

 

I have a 94 Peterbilt 379. Must have came from same person! Wires cut and ran to other switches too! Good luck.