04 387 side lights not workin on truck and trailer any help can't find a fuse for it

Most likely you have a short in your Tractor wiring, there isn't a specific fuse just for the side lights. There are separate fuses for Tractor Marker Lights and Trailer Marker Lights.

If a specific section of your lights are working, but another section isn't working, it's usually a short in the wiring somewhere.

Since some of your Marker Lights are working, you can rule out the possibility that you have a bad fuse.

 

Do all of your Tractor lights not work when the Electrical Cord is disconnected from your Trailer or does it happen when it's plugged in also?

 

Then you have a short in your Tractor, most of the newer model trucks have what they call a LCM (Light Control Module). If some of your lights stop working and it also affects the Trailer, most likely you need to replace the LCM. I would call a Peterbilt Dealer and ask them if your truck is equipped with one.

 

In that case you need to start tracing where the bad wire is starting from the point where you know the wiring harness for your lights has power and go from there. You will need a Test Light Probe.