A while ago my left rear window stopped working in that it would go down, but wouldn't go up. It would just making popping noises from the gears not fully engaging. Now my other rear window is beginning to do the same. When the first one went bad, I suspected the motor or regulator, but they both looked fine, and with the second window I've noticed that it slants a bit when I try to put it up and that all I have to do to engage the gears is to realign the window with my hands. If I do this while holding down the button, the window will go up just fine.

I'm guessing this is due to the window slide (little plastic thingy that I believe attaches to regulator arms). Are there any other alignment parts besides the slides and felt/rubber window tracks that I should check?

Hopefully fixing one window will provide the key to fixing the other.