Check fuses and relays, if everything is fine there's probably a short somewhere. Maybe also pull your splash guards down from the rear wheel wells and reach up and check out the couplings for the tail lights. Maybe they're dirty or corroded. When I installed a trailer harness on mine (it bypasses the harnesses for the trailer) mine were a little bit dirty. Could be that.
If you're talking about the tail light on the tire mount check the terminals at the rear of the cabin and on the tailgate, sometimes your carpet can get caught or they get a little surface rust and won't work properly.