I am not following your tests. If you are using your voltmeter correctly, you should not have any voltage on ground wires. They are supposed to be at "'ground", meaning 0 volts.
What I would start with is going right to the window motor's connector. Unplug it, then measure the resistance from each wire to ground. Ground your ohm meter somewhere on the body or right to the battery, not on the door. Both wires should read very low resistance, unless you have GM's famous design with relays built into the switch. For that I will have to dig up a wiring diagram to be sure I am telling you the right things.
If both wires read good continuity to ground, switch to reading voltage, then measure each wire while activating the switch. Only one should get 12 volts in the "up" direction, and the other one should get 12 volts in the "down" direction. If they do, plug the connector into the motor, then measure both wires again.
If you find 12 volts on both wires when the switch is activated, there is a break in the ground circuit that got overlooked and we will have to pursue that.
Saturday, August 13th, 2016 AT 11:17 PM