If my book is correct, the power windows, power seats, and power tailgate share a single 30 amp circuit breaker. (After the breaker, the one wire feed splits to three!) To test, replace breaker with a 30 amp fuse. If they work, replace the faulty breaker. (Check ALL the breakers. If one is shot, the others will probably fail soon.) If this doesn't work, check for power where the breaker plugs in. If no power, check under the hood for a wire with bubbled insulation. (It'll be near the battery. Trace the positive cable to the starter relay, and it should be there.) IF all the above checks out, check the switches. All the window and seat power routs through the driver's door window/lock switch panel. Tailgate switch is seperate.

1973 J-4000 pickup "custom"
(built-up EFI 4.0, SM420 4sp, Dana 20 t-case, Dana 44 HD front, Dana 60 rear)