Unless you are willing to remove the interior skin I doubt you are going to PULL new wires. The wire run through the ceiling and pass through several ribs.
I would remove one of the rear marker lights and check the wire color feeding it. Then try and find that wire forward and connect FUSED power to it as a test.
Remember you will have to reattach the marker light for the test as the light body is the ground path to the trailer skin.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles