Since you already have the door panel off, try attaching a ground wire to one of the motor fasteners. Run the wire through the boot and find a ground spot somewhere on the dash frame or firewall. There is no ground wire for the power windows. The motor is grounded to the door via the fasteners and the door to the body of the car via the hinges. A really poor design because those areas can rust. So, if you run a new ground wire, you can eliminate a bad ground. That may not be your problem though. You should measure the voltage at the motor and switch.
They had a thread on this on that "other" forum which shall remain nameless! The summary of the post was that the design was poor, not properly grounded, and there was too much current drawn through the swtich which causes them to burn up. The poster recommended a new circuit design which took the load off the switch and enhanced the grounding. I took his narrative and drew up a crude wiring diagram. Essentially, you bring in a new power supply line from the battery and hook it up to four relays, one for drivers up, drivers down, passenger up and passenger down. The switch then controls the relay and the relay takes the load. Each relay is grounded as well as both motors. This takes the pressure off the switch and provides sufficient power to the motor to power the windows up and down. If you want the diagram and narrative, send me an email and I'll send it to you. The poster did a great job in describing the circuit....so good that an accountant could take his words and draw a picture.
Sorry for the long post, but this information may be helpful in the long run.
