Hi Al,
The regulator uses two arms to move a set of white plastic sliders that each have their own metal track at the bottom of the window. It's quite common for them to break, either because the window is under too much stress, or the plastic fatigues over time. The metal clip that retains the ball jointed regulator arm in the plastic clip can also pop out and disjoin the two.
Remove the door panel and have a look.
I'll get a few pictures up...
Alright, photoshop is acting up, so I didn't edit these down and they're about a meg a piece.
When reinstalling the clip (#3), use a set of channel locks to seat the clip -- if the clip isn't fully seated, the jointed end will pop out of the plastic slider within a few lifts of the window.
http://www.bimmer.info/~lowell/tr1.jpg
http://www.bimmer.info/~lowell/tr2.jpg
http://www.bimmer.info/~lowell/tr3.jpg
http://www.bimmer.info/~lowell/tr4.jpg
best, whit