First look at the fusebox to check which fuse
controls the windows. Check if it looks intact -
there is a small strip of metal along it, that may
be fused. If it looks ok, rotate it a few times.
If you have a spare, replace it.

Other thoughts - trace the wiring from the motors.
I had an MBZ whose seat would not move and I
traced the fault to a bad electrical connection
under the seat. Try to find a wiring diagram to
show you where the wires and connections are. A
small multi meter should also help - will tell you
whether a wire is live or not.

Good luck!

corker schrieb:

Since there two fuses (a,b) for the four windows and four switches
involved: If all four windows dont work it could only be the common
15-permanent battery connection ein the fuse box or more likely the
common mass connection. It should be besides the switch group.

All motors have bidirectional wiring with switches interchanging battery
and mass connection. So the mass connection is at the switches, not at
the motors. (Extrapolation from W123 diagram)

It is almost always the switches that needs to be taken apart and cleaned
up. I have never replace any relay for any MB I have or worked on.  Take
your time to take it apart... you will see how simple it is to do this job.

Either way, clean or new switchs, you have to learn how to get them out.

I had the same problem with my rear windows. My mechanic cleaned the
motors and they worked fine for a while. The last time one of my windows
got stuck in the down position. I then replaced both rear motors and
mechanisms with new ones (~ $200 a piece without labor). Everything has
worked smoothly since.

They work much smoother than the front windows that were not replaced,
these seem to struggle a bit to close and open things. I had no problems
with switches.

X.