First I would study how they wired the parking lights. If they tapped in directly at the rear light fixture they may have just missed the directional connections. You will have to pick up and wire each side separately. Brakes and directional are the same at the rear of the vehicle.
The only wire that needs to go forward is the battery charge circuit the others can be tapped into at the rear lights.
Hopefully the harness to the plug at the hitch is color coded so you can follow it. If not test each location with a meter to find the 3 and 9 o clock positions, directional locations.
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