trailer lights

[Waterloo]

Sounds like a ground issue. When this happens to me, if I cannot visually see something, I run both the truck and trailer into a shop with one of those trailer electrical diagnostic tools. It could be anywhere in either your truck or trailer(s).

Also, many trucks have a grounding strap from the tractor frame rail to the fifth wheel. If that grounding strap is broken, or lose, it will give you issues like you are having. I had this happen on a new Pete once, drove us nuts for a week or two. Trailer shop figured it out in around ten minutes. The fifth wheel grounding strap was still connected, but not making good contact. The reason I like to take both units in when issues like this happen. A good trailer shop has the diagnostic tools to figure this stuff out, beats crawling around under the truck and trailer chasing wiring for hours on end.