Yo,
Latch Safety Switch (also ref to as Limit Switch or Interlock Switch); "...miesk5 Note; In 78-82, both latches have to be closed since the interlock safety switch is mounted in the center mechanism; & their strikes that mount to the body can be interchanged with some effort. The safety switch was moved from the center (tailgate handle) to the left latch by 1983 by Seabronc (Rosie, Fred W)
If the latch doesn't lock, try to get it to latch with the t/g down and use a screwdriver (or something slightly smaller than the strike post) to close the jaws. You probably just need to rinse it out with Liquid Wrench (not WD-40; too much wax) or some other penetrating oil. If it still won't latch, put the glass up & pull the access panel. Inspect the operating rod that runs from the center mechanism to that latch to make sure it's not bent, keeping the latch released. If the rod is SLIGHTLY slack and the latch is clean & lubed, but won't catch, replace the latch.
If it latches with the t/g open but not EVERY time that you close it, trace a circle around the big washer (for reference) and then shift the strike post rearward ~1/32" at a time until it latches or you've gone 1/4". If it still won't latch, then you need to move the post up or down to align it.
"...The safety switch was moved from the tailgate handle to the left latch by 1983..." per Seabronc; It keeps the window from being moved when the tailgate is open, & if not latched right, it'll keep the window from working at all. Try pushing on the tailgate in while holding the button or key switch in the up or down position. If that makes it work, check out Steve83's How to align a Bronco Tailgate linked below. Ground for the motor connects up front, & at rest runs from there through both switches & both wires to the motor. When either switch is operated, 1 side's ground is disconnected & replaced by hot at that switch. Ground for the motor is not connected to it's case, but supplied through the wires. The 78-79 TG ckt included a 20 a Circuit Breaker @ starter relay & receives 12 v from Ignition Switch; whereas later years don't have the CB there; it is of course in the Fuse Panel or have 2 fuses in the panel; & Relay Location in 78-79; for the 78-9 People, for us, it is quite easy, under the dash behind the instrument cluster, there is a relay, that relay is to run the main constant power for the rear window when the key is on,by origin at FSB; Relay Location in 78-79; Behind the guage/speedo is a relay that powers the inside switch, check it, by Zebrafive at FSB;
Testing: Does motor make any noise at all? Like the motor's turning, but the window doesn't move? If so, there are torque pins in the motor's drive gear that commonly break & aren't too bad to replace. See the torque pin, Steve83's & Adrianspeeder's links. If it makes no noise, then it's probably in the electrical system.
Sometimes the connector for the tailgate motor will test ok, but not make contact when plugged in.
It's possible for a damaged wire to carry enough load to show 12v on a tester, but not enough to operate the motor. Look for worn areas, especially in the loom between the body & the tailgate.
If motor runs, but doesn't move when wired directly to the battery, in addition to your electrical problem, the torque pins in the drive gear are probably broken too. .."
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