The motor runs off two wires. It works in the other direction if you reverse the polarity, which is what the switch does. Each switch has power and ground going to it and the outputs are power and ground. The outputs are reversed to change direction. This may be called double pole double throw. Internally there are two switches moving.

This needs to be studied on a wiring diagram first. I have fixed hundreds of these and my first step isn't the door panel. It is the wiring diagram.

But, if you are in the door, check power and ground at where the motor wires are screwed to the harness. Check both sides of the circuit!!

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician