What you have described is puzzling. This what I have gathered:
- Neither switch works on passenger side. If they work on driver side, then that should eliminate the switches as the problem.
- Window motor runs and moves window up/down when 12V (+/-) is applied directly to the motor. So motor appears good.
- With switches at rest, there is 12V at each motor terminal. That is as it should be.
- When switch is actuated in up or down position, voltage on motor terminal goes to zero. This is the puzzling one!
What would happen if ground connection (6) is bad? You would have an open circuit and motor wouldn't run. But that wouldn't give you 0V to ground.
Question - when you are measuring voltage at motor - how are you doing that? (You should have one side of meter on either motor wire and one on a good chassis ground.)
Don't know if this will help you, but this is the wiring diagram. One thing you will note, is that the driver and passenger side front are on different circuits with separate fuses. Does the driver side rear work normally? (it is on same fuse as passenger front).
How about the passenger rear? It shares the same ground as the passenger front.
From this you can see that if there is 12V at connector C107 (5), then you will have 12V to ground from either side of motor connections (4 and 3). As Tango said, if you push switch to either up or down, then you will have 12V still on one terminal (say 4), but the other (say 3) will be connected to ground. Now, if the motor is stalled and does not run, then the motors internal breaker will open. But motor ran when 12V was applied directly so that is unlikely. Confusing!