99 Peterbilt 379 Wiring Issue

We have a 99 Peterbilt 379 with a 3406E Cat. The issue we are having is in regards to marker lights. When hooked to one of our trailers, all the lights work, even the side skirt & breather lights. When un hooked from any trailer, the only lights we have are the headlights. No clearance, no tail, no side skirt & no breather lights. When hooked to our second trailer, still no lights except for headlights. Not even on the trailer. Turn signals and brake lights will always work on both unit and trailer regardless. Just nothing else. Any help is greatly appreciated. Was going to start chasing wires this weekend!

 

Will do thanks!

 

That's correct. When hooked to trailer A everything works on truck and trailer. Hooked to trailer B, only headlights. Unhooked from both, only headlights.