It's not the fuses if both back windows work. The windows are cross wired: one fuse controls left front and right rear, the other controls right front and left rear. Never understood why, but maybe it was done just so I could say "definitely not the fuses".

What you're going to have to do is remove the door card. There will be a two wire juction somewhere on the door panel. BOTH contacts should be 12v+ without touching the switch. BOTH of them. If one or both of them are cold, then that's one problem.

Have someone hold the switch in the up position. One of the door contacts should remain at 12v. The other should go to GROUND. That's right, GROUND. If your assistant flips the switch to the down position, the polarity will reverse: the ground will be on the other terminal. If this is happening as I've described, then your problem is the regualtor motor. If this isn't happening, it's the switch.