This is a known problem with the W140...happened to both of my rear windows
even though they were hardly used.  The regulator assy has a critical part
that's made of plastic, and it just breaks.  The replacement part uses a
metal piece.

I replaced both of mine myself, takes less than an hour per side.  If I
recall, the part costs +/- 150$USD from the stealership, I could look up the
receipt if you need exact price.  Aside from a screwdriver, you'll need a
drill to drill out 3 big stainless rivets, and a big pop rivet gun to
replace them.  (I can't remember if the rivets came with the regulator assy,
or if you have to ask for them separately...make sure you get them)

The hardest thing was to figure out how to get the door apart.  I found
directions on the net somewhere.  One note...The first step is to pry the
little "half-oval" shape piece of woodgrain plastic out of the door.  Be
careful !!!  The woodgrain show surface is about half a mm larger than the
backing plate behind it.  Why you pry it up, make sure your tool is in far
enough behind the part to pry against the backplate and not the little free
edge of the show surface.  If you don't, you'll end up cracking the show
surface and replacing that part as well.  I wasn't and managed to crack them
both.

Good luck...John