Did you or the P/O mess with the wiring inside the tailgate? Because the two wires going to the motor are both 12v (one makes it go up and one makes it go down) and the case of the motor is ground. Other than that I would say either the switch in the gate or on the dash are bad with an emphasis on the gate switch.
And if you still have the old one, I'm assuming it doesn't work, is there any chance I could work out a deal to get that from you?