Not sure the 99s came prewired. My 97 had the wiring in the harness but did not have the receptacle at the rear bumper or a connection forward.
Take a look under the hood just below the steering column for 4 wires folded back on themselves and taped. Also look inside the drivers side rear frame member. There should be a hole on the inside of the frame about 15 ins. from the end of the frame. The wires will be inside that hole folded back inside.
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