06 Volvo, all lights work on the truck, to right turn signal on trailer.

It sounds like the problem could be the wire between the fuse box and the sleeper plug. On mine it looks like a standard green insulated pigtail cord. Try running a new wire from the back of the fuse box to a test light. If it works, you could run a new wire zip tied outside the old sheathing, from the fuse box to the socket under the sleeper. If it doesn't work, I'd look at the pins and connectors next.

 

Did you ever your issue my truck is doing the same exact thinh

 

It was someone else's issue, so I'm not sure if or how they solved it. Grab a multimeter and start testing for voltage and work towards the signal. Wherever you're not getting voltage from is where the problem should be. If you get adequate voltage all the way to the light, you likely have a bad ground connection or bad light.