On my Challenger, there are two places where the light can mount.(front side of the cross beam and back side) Have a look at the holes in your car and see if you can see where screws have been mounted previously. On mine, you could tell that screws had been installed on one side and not the other.
I ran the wiring across the beam and into the roof structure. There were one or two clips that held the wiring to the roof beam. Then, the wiring came out under the rear interior panels and connected into the tail light harness in the wheel well area.