Rear tailgate window does not go down

Truck listed above is the Silverado model. Rear tailgate window won't go down, have replaced motor, cable and bypassed safety switch in drivers side tailgate latch. Currently works from front switch, up only, rear key switch works, up only. At times the rear tailgate motor, without cable in sounds to be working both ways but not always. Cleaned connecting union coming to tailgate, no help. Rear safety switch is no longer made. Chevrolet house said it's obsolete. Looked all over Internet, no luck. But don't know if that's the problem. Currently working window up and down with electric drill, so it does work just not electrically. Any help would be appreciated.
Thanks, Charlie