Do you have an umbilical cord on the trailer with a 7 pin plug on it? Do you have a 7 pin receptacle on the truck and a brake controller in the truck?
If you have the truck wired take a volt meter and read between the 2 lower pins on the truck receptacle while someone steps on the brakes and then moves the manual handle on the controller. If you see voltag in each of these 2 experiments the truck is fine.
Connect the umbilical cord. With a compass held to the wheel and close to the bottom of the brake drum see if the compass needle moves when someone applies the brakes. This will test the complete electrical system of the brakes. It does not mean the brakes are adjusted correctly or the mechanical parts are good. Removal of the brake drum and visual inspection is next.
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