I had the same problem with my grand prixs rear windows. We bought the car for 800 dollars off some painter trying to get rid of it and had to drop an engine in, but everything runs fine.

I finally got the time to check out what was wrong with the windows; pulled the panel off, and there was nothing even inside.

They cleaned it out. So I just c-clamped a piece of plastic in the track and let it be. lol