Here's a circuit diagram of an MX62/63.
The later models will be pretty similar but the wire colours may be different.
The ground and +12V both go through the master control on the driver's door, then to each passenger door, then to the motor.
With hands off the switches the motor is grounded on both wires.
Pushing the driver's switch or the passenger's switch up or down feeds +12V to one side of the motor (the other remaining at ground) and so the motor moves.
Possible causes are:
- faulty driver's switch
- faulty passenger switch
- faulty wiring between the driver and passenger switches (light blue wire marked as L is ground and will be okay)
I had the same problem on one of my MX73 rear doors.
It went down but not up - a real bummer when we go to the local supermarket.
Replaced the rear passenger switch and now the rear switch doesn't work at all but the driver's switch works fine.
At least I can wind them up now when I park the car at the supermarket