Working on my mother's 08 Cruiser base model.

I've replace 3 out of 4 window regulators/motors within the past year as they stopped working. I've tested each one to confirm that the system was working properly and that it was indeed the window regulator. All 3 worked properly after replacing.

I figured it was a matter of time until the 4th one finally died too. Well, it finally did.....or so I thought.

With the window motor disconnected, testing just the connector inside the door I get +12V circuit going one direction, -12V going the opposite direction. It tests properly with both the front switch and the rear switch. The window system tests fine. Just like it did when I tested previously to confirm the system was working properly. So I ordered a new regulator/motor.

Installed the new regulator/motor and it's not working properly. The window will go down, but not up (just like the old regulator/motor, and why I ordered a replacement). I probe the electrical connector as I'm hitting either switch and it tests fine: +12V one direction, -12V the opposite direction.

Same problem, 2 different regulator/motors, one the old original and one OEM new.

The connector inside the door is not corroded and doesn't have anything visually wrong with it.

Plugging the new motor in and testing for voltage drop across the connector in the door checks out fine with nearly identical readings on both sides of the connector for both wires.

If I use a battery jump-pack and connect it to the new regulator/motor directly, I can get it to go up and down smoothly with no hint of binding or resistance, which would increase the electrical resistance and could be the cause....but it checks out too so that's not it.

Clearly there's a problem with the system (not motors), but I'm kinda at a loss for what the culprit is and what else I can check to verify.

Please, please, pretty please throw out some ideas for me!!!