Here's the pics of my third brake light wires!
First I drilled a hole in the back of plate were the tire mounts!
Then I ran the wires down the swing inside wire loom and zip tied to the arm of the swing!
I left a little extra near the hinge so it would open without interference!
Then I went up inside the inner fender well to tap into the factory wires!
That's just how I did mine, others I think have spliced into the wires that come out from the tail gate! Hope this helps and good luck!