I would check the grounds, I am not sure how many are in that circuit but I would guess there is at least one near the tail lights.

Electrical gremlins can make you crazy. I had an issue with my wife's old 97 S10 Blazer where the interior lights would randomly go on and off. It ends up it was the lumbar control switch in the driver seat, as you would move around in the seat the wires for the switch would ground out on the seat frame and since the interior lights and lumbar (possibly all of the power seat) were on the same fuse grounding the lumbar switch caused the lights to come on. I probably never would have tied the two together if I hadn't had noticed that I could make the lights come on by moving around in the seat.
Long story that has nothing to do with our issue I realize other than to say good luck.