I have a 99 GC Sport with the left tail light out. I've checked bulbs, swapped them out with known good and still no tail light on left side. Brake and Turn signals work fine. Checked the connector and there is no power there so its not in the tail light assembly. Last summer I installed a hitch and trailer wiring adapter and everything worked fine but some where along the way I lost power to that light. I've disconnected the trailer pigtails and its a problem somewhere in the van.

is there a fusible link or something in that circuit that could have been fried by the trailer lights, or is that just coincidence? Any thoughts short of tearing the interior apart and chasing the wires through out the vehicle? All other lights and electrical seems to work fine.

 

Check the splices where you tapped into the factory harness for bad/loose wires and corrosion.