I would stay away from the bumper area. I have a LoD bumper with the floods mounted in the bumper. It seems the heat from the muffler melts the insulation on the wiring and it shorts out. Your wire insulation options are limited, as this area also gets wet. I would avoid running the wiring in the bumper area all together. Higher mounting the better I think. If you do run the wiring in the bumper, use THHN, it's 90 degree C and wet locations rated.