Tail light fuse keeps popping out. 1994 Peterbilt 378

You got a short get a multimeter track it down and fix it

 

One way to find it, if you have a dead short, would be to take a LED light or a light that has low current draw.
Take the blown fuse out and replace it with the light, if LED make sure + end is on the hot side of the fuse the other end to the other side of the fuse. If you have a dead short that light will illuminate. Now have someone watch that light will its on, go down the wiring harness of the effective circuit, what the fuse protects, and do a wiggle test. Just move the wiring harness around when the light goes off you have found your short. Hope this helps.

 

Buy or borrow a pigtail, it takes the most abuse.

 

Do you pull a trailer?

 

Do you mean the tail light circuit breaker? The white button pops out?

There is a short somewhere on the #8 circuit. That's your marker lights (top of the cab are clearance lights circuit #5)

The headlamp relay gets power from that circuit so if it's out you won't get high beams, just low.

Outside the truck you're going to have to trace the wires. Hood hinges are great for crushing the headlight harness and causing shorts. Check there first, and it may not be obvious without pulling the loom off the wires. You can unplug the headlight harnesses and see if that fixes it.

Inside the truck you can pull the kick panel and look for burnt wires/crushed wires. I'd bet that it's outside the truck though.

 

Unplug harness plugs one a a time it help isolate the problem. I do not thank it is the head lights.