Have you jumpered the motor leads? If not, that is one of the the first thing I would do just to make sure it functions properly. Put the Plus lead to one motor lead and Minus to the other motor lead and then reverse the leads to reverse the direction of the motor. (Do this with the motor mounted)
If that works, try pushing in on the left side of the tailgate while you operate the tailgate switch. If that works you need to adjust the latch.
If that doesn't work, jumper out the latch safety switch which is located in the left hand tailgate latch. A little hard to get at but doable.
NOTE: the ground for the motor goes through the driver switch. The driver switch must be in the center position for the tailgate switch to work. (Also, the wires from front to back do double duty. Either lead can be (+) while the other is (-) when operating the tailgate from the driver switch. Both serve as grounds when operating the window from the tailgate switch.)
Points of failure may be any connector in the front to back path. Corroded connections, broken leads (where the cable flexes, or corroded and broken at the connector). A prime offender of corroded leads is the connector just behind the left side of the rear bumper on the rear cross member.
Good luck,

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