Process of elemination:
If stops on trailer do not work when vehicle switched to headlights but do when vehicle on side lights then it's auto electricians job.
If trailer stop lights do not work in tow vehicle side light or headlight mode then check current is reaching socket on car for stops. then the trailer cable plug then the trailer lights clean terminals and bulb terminals.
Another test would be to plug the trailer in to another car tow socket to rule out the trailer.
www.tridenttrailers.com/trailer-wiring-diagram
bob, April 2009
