I'll admit this problem is in my 90 C2500 but the systems are pretty much identical as far as wiring goes. My passenger window doesn't want to go down, and I know exactly why just not sure what's causing it. For whatever reason I'm not getting a negative. Positive and negative are fine for up just not down. If I feed a negative to the blue wire, even from the door, it'll go down just fine. It's not the switch, I've swapped every switch to find the same problem. Same issue controlling from both doors. I'm baffled.

 

I would suspect the door lock and window switch driver if the problem is the same from multiple switches on both sides of the vehicle.

 

Nice, that's a schematic I haven't seen yet. Thanks! Don't suppose you know where said driver is located before I go tearing my dash apart looking for it?

 

Both wires at each motor default to ground. When a switch is activated, it removes one of the grounds, and applies voltage. Start probing wires until you no longer show a ground.