I dont know what year your jeep is but if its the same as mine...(95)
Open your tailgate and look down where the hinges are...you will see 2 little nickle size round pieces...theres 2 on the tub and 2 on the tailgate...these touch when you close the TGate.
if you extract those,thats where your wires are....the 2 on the Tgate run up to the 3rd brake light...2 on the tub run to the wiring harness..i think. lol
had trouble with the 3rd light not working...ended up being when i bed lined my jeep i accidentally sprayed over those 2 metal thingies lol..cleaned them and it worked like a charm..hope this helps