wnjj wrote:
The tail and brake are different lights or at least different filaments in a bulb. The point is if it's the tail light, the other side and the clearance lights are wired in parallel to it.
Since it's by itself, it's likely the brake/turn light (same filament).
I'd check inside the 7-pin (or whatever Lance uses) for a strand of wire that is bridging between the charge pin and the left turn pin. If it's the Lance plug, a diagram I found shows the left turn and +12V are next to each other on the bottom.
As I think about it, the original owner (re)wired the umbilical from the truck. I bet that's where the issue is.