Window stopped working

[idriveaboxster1]

One morning a couple months ago, I got in my 1997 boxster and opened the door to get in. When I tried to shut the door, the window got in the way of the upper sill. when i pressed the window button to go down, it moved about 1 cm and then made a high pitch noise and buzzed, and then stopped working. Since then the window has not moved. It makes no sound when I press the window button. This is only the driver side door. Passenger window still works fine.
I took the door apart to see what was going on. I took out the motor for the window and tested the wiring harness. It was receiving electricity when I pushed the window control button. Then I tested the motor separately. It worked fine when current was applied directly to the leads on it. I thought it must be the small control unit attached to the motor. This contains a rather complicated chip which receives power from the 8 poles on the wiring harness and puts out power on the 2 motor poles.
I ordered a new motor because I assumed this control unit was broken. When I got a new motor, I installed it and the window still did not work. This really confused me. After this failed, I sent the new motor back.

Any ideas on what the problem may be?

Thanks

Paul