I had the same problem recently on the driver's side. It was the regulator so I replaced it using the Bentley manual. It took about 1 1/2-2 hours and wasn't bad.
If you do it, check with your fingers and make sure the forward part of the glass is actually down in the clamp that's on the regulator. Otherwise you will waste another hour or so like I did trying to figure out why the darn thing wouldn't raise and lower properly. I thought it was in, but when I stuck my finger over there it was sitting on top of the clamp.
Good luck!