I've done this job on the driver-side window/door of my '91 420SEL (W126), and it is not too bad. But I think the 1-1.5 hour estimate is low, if it is the first time, if for no other reason than taking the door panel off (and getting it back on right) takes a little playing around.

In seeing lots of forum postings about similar problems, I think it is always the teeth on the regulator, and not the teeth on the motor, which are probably made of harder material.

I think it was this forum where I did a search and found a nice posting from someone with pictures on removing the door panel, separating out the regulator and motor, etc.

I got my replacement regulator from an online parts place - for my car there was a need to be a little careful in ordering, since a couple of different makes of regulator and motor were used, and you had to get the right regulator to match up with the motor. Just something to check.

The problem with this is that once you take everything apart to see exactly what you need, you are more or less stuck with an immobile car until the replacement part shows up (since you probably don't feel like doing the whole job twice). Or you can make as assumption that you need the regulator, order prior to taking everything apart, and just hope that you don't end up having to return anything.

My only regret after fixing mine was that I lived with the clunking sound (in my case the window went up all of the way, and it just made the clunking sound after the window was at the top of its travel) for almost a year before getting around to fixing it.