You might check the connections for the trailer harness. It will be across the back and I think it will have a plug on each end and the trailer wiring in the middle. I would unplug all the plugs and plug them back in. See if you can find any problems. Make sure the vehicle's turn signal is working on that side.
It would have been easier to find the problem if it was the trailer wiring. That's the bad news.
'96 ProStar 205 SD LT-1