I'd guess a ground issue.
Check continuity of truck side receptacle ground wire to truck.
If good, check all the + wires on the truck side.
If good, check a ground wire on an individual light back to truck (plug/frame etc. I beleive TC lights aren't grounded to the camper, but back to the truck, since they are insluated from the truck (electrically speaking) it would create a ground loop if the lights werent grounded back to the truck (the source))
I've not tore into TC running lights before, but given the lack of propensity for metallic surfaces near all the lights, I'd think each light has a ground running to a common location, all spliced together and then 1 ground to teh truck.
