My '87 W124 250D (5 cylinder, 2.5 liters, do I understand correctly that
these were called 300D in the US?) had the same problem: rear right
window stuck. I noticed it starting to fail intermittently before it
finally stopped altogether. The motor seemed to give a faint click when
I rocked the switch. Swapping the switches didn't make a difference.

I took it to the (independent) garage this thursday. The mechanic took
the motor apart and cleaned the brushes/collectors. The window now works
even better than the others. Just cost me the labor (not sure exactly
how much, also had some motor mounts replaced, windshield wiper motor
fixed and regular maintenance at 140 EUR labor).

Ximinez

On my 80 300SD, 3 original ones are working fine.  The one that failed
was the driver door.  And that failed because I made the mistake of
trying to open it when it was very cold outside and it had gone through
the car wash earlier.  The replacement I got lasted about 5 years.  Few
weeks ago I replaced that one again, because it too had failed.   The
plastic bushing in the regulator arm slide was shot.  Unfortunately,
that is not replaceable as a seperate part.  (Or if it is, I can't find
one anywhere).

The replacement ones are not as good as the originals.  I guess you
could get one from the stealership.  The first replacement one I got
was from an online parts store.  I think I paid around $140 for it.
As I said, it lasted about 5 years.   This one I got on Ebay for $75.
I've since seen them on there for $50.   When I put this one in, after
getting it all lined up, which ain;t easy, I tried to put the bolts in.
One wouldn't start because the threads were not cut right.   I didn't
think I even had a 6mm tap, till I looked.  But I did and then I
remembered why.  The one I got from the parts store 5 years ago had the
exact same problem and that's why I bought the tap!  So, you can get
the same thing on Ebay or pay 2 times more and get it from a parts
store.

I did spend more time fine tuning it this time.  I think part of the
reason the replacement one failed last time may have been that it
wasn't adjusted correctly.  This time I made sure it's perfectly
aligned.  I also made use to thoroughly coat the arm slide with grease.
She goes up real nice now, faster and smoother than the originals.
Hopefully it will last.

As a side story, when I did this job, I temporarily unfastened the
vacuum switch on the drivers door.  This is the vacuum switch the key
moves to activate the central lock system.   I finished the door job
and had it all back together and next day went shopping.  When I came
out, the other doors did not unlock.  So, even though all I did was
move this thing out of the way a bit, never even unhooked vacuum hoses,
you would think something I did must have screwed it up.   So, I take
the door apart again and start tracking it down.  Turns out the vacuum
line going to the door from the engine compartment had cracked right at
the firewall.  Had nothing to do with me taking the door apart, pure
coincidence!

As a tip, this is the second vacuum line that has cracked at that exact
same point.  The other was the engine shut off vacuum line.  That line
made a sharp bend as it went through the rubber grommet, which I
assumed was the cause of the failure.  This one went straight through,
no tension on it at all.   So something else is going on, whether it;s
vibration or maybe because there is a steep temp change at that point
from the hot engine compartment, etc, I don't know.  But if you have a
vac problem, that is a good place to look.  When I pulled on the bad
ones, they just broke off.

and later, I repost the same thing for the back two windows...

Have had both on cars.....but I also have long arms and can roll down
the right window without any problem.

Advised my SO to get manual window's on her mini-van, but she now
wishes she had power.  Mopar van power window's have a nylon "tape"
that breaks. (have changed several of 'em)

Changed the regulators in the SL, cause of the teeth being chewed up,
another broke due to driving the washboard backroads of Arches NP