How do I test a power window motor
 

'90 100

There are 2 wires that go to the motor housing from the switch. Is it simply the current direction that defines which way the motor goes or is each wire a direction and it grounds through the body?

On a two wire motor the polarity determines the direction, however
 

I have never taken one out of an Audi and tested. All of the two wire ones I have seen work that way. I would not be surprised if some manufacturer used a common ground to perform this task but the motor would need two windings to do this and I don't think this would make design sense for a DC motor. If you have an Ohmmeter you can check for open circuit between the case and each of the wires. On a good motor this would mean that the windings were both isolated from frame ground. You could also check and see if the polarity of the power from the switch reverses when the switch is activated in each direction. Put a test light (or a DVM) between frame ground and each wire, the light should light on one wire and not the other when the switch is activated in one direction and light on the other wire when the switch is activated in the other direction. If this is the case and the connections to the motor are good, your motor is NG. I use a battery charger or a 12V power supply to test them, the motor should spin in each direction when the polarity is reversed.