Pete 386 Weird wiring issue

Sounds like a bad ground

 

I had similar problem but it was in my trailer. Left signal would flash all lights, same with breaks. It turned out to be bad ground in the pigtail was causing it. But like i said it was on trailer. I would imagine u have a bad ground somewhere on the truck.

 

I would start by disconnecting harness one at a time. Each rear light. Any connectors in the harness down the frame? Then move to the front.
Had a 388 with a bad chassis control module. It was behind the trans on the left inner frame rail. Believe all the turn signal and markers run through that. You may or may not have one.
That truck had a harness rubbed through on the rear cross member which probably damaged the module.
I would try that with trailer cord connected and disconnected. And yes check grounds.

 

Mechanic here. I have a 2009 Peterbilt 386. In our shop very similar issue.
Ive noticed alot of lighting issues with these and Kenworth. Both have a module (chassis node) on the frame has three mounting bolts and three electrical connectors. Changed it and bingo. Unfortunately there very expensive. 1400 plus..
Jimmy

 

Work on these all the time . Theres a chassis node on the inside frame rail on driver side. Has three mounting bolts and three electrical connectors. Start theres very common and expensive. Find another working one and try it hope this helps