looks like they may have daisy chained the marker lights. if when you removed the old light all 3 wires were connected together that is the case.
To test it out. Turn the parking lights on. Are any of the marker lights to the rear of this light lit? If not I think they are daisy chained. if you have a 5 amp fuse use it to touch the 2 wires coming out of the trailer together. While this is in place do the marker lights to the rear of this light come on. If so discard the fuse and connect all 3 wires together and you should be done.
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