I tried fishing wires through a tail gate before, and it did not work. There is some type of tight spot in there (lower left side), which won't allow it (or makes it extremly difficult). After thinking about it for a while the only solution I could come up with was to drill out the spot welds, which holds the tail gate face plate on to remove it. After the wiring was done then the tail gate face plate could be screwed back down with sheet metal screws (or riveted back on). Maybe you could use a small hole saw to drill a hole in the lower left side, which would allow you to fish a wire past that tight spot. Luckily, the tail gate wiring on all of my 1 tons, I have and had in the past, was good enough that I did not have to replace it.
