He said the rear of the roof.
Use clear RTV and not black. Black is for losers.
I can't say that I've had a rear clearance light off, but I assume it has the same grommet as the fronts. Problem would then be fishing the wire from the ceiling.
Is it going to the front anyway? I would try to put it under the rubber or alongside the roof-rail tracks and run it inside at a front marker light as ree shows.