Right Tail light/blinker is out when trailer is hooked up to truck. I tested the plug on the truck and all leads work. Used a battery pack to test all leads on the trailer, and everything lit up normally, INCLUDING Right tail light/ blinker. HOWEVER, when I plug the trailer to the truck, the Right tail light blinker doesn't work. All other functions/lights on the trailer work fine. Anyone have any ideas? Thanks in advance.