peterbilt lights blew breaker?

try unhooking the pigtail than see what happens, would rule out the trailer atleast. had something similar happen and it was actually in the pigtail on the tractor side, some wires got chafed and was touching causing some weird problems.

 

im not totally sure if it will work right but being a 379 im sure you have 2 different switches for headlights and clearance lights. maybe try turning only one on at a time. example...just turn headlights on and see if the breaker kicks, can rule out clearance lights i would think. so if it still kicks with only headlights it would be headlights, marker lights, or taillights.

just throwing this out there but try unplugging the pigtail on the tractor side instead of the trailer side, thats what i finally did and my problem was solved. im sure somebody smarter than me will eventually come along but thats what i would try, just throwing some ideas out to ya.

 

My 359 did this, it was a wire that disconnected at the pigtail on the trailer and was grounding out. Being that yours is doing it without the trailer means you've probably got the same thing going on but on the truck. Check the wires around the hood hinges, just caught a blinker wire doign the same thing there on my truck.

 

First thing I'd check is the license plate light on the rear of the tractor. If not, I'd unplug the rear light harness and see if the breaker still pops. If it still does, unplug the next set of light on that circuit and retest the breaker, and on and on until you find the problem. This is simply troubleshooting 101.

 

I got the same problem with tail lights on back of the truck keeps blown breaker 2011 peterbilt 387