This is a common problem that can be somewhat challenging. This tip applies to almost all models with power windows from the late 1960's to the mid 1990's. Window regulator motors often fail when the window is closed! When this happens you can not easily lower the window to disconnect the regulator from the bottom of the glass. So you are literally STUCK!
How to get the regulator out the door when this happens
W111 W112 Coupe
You really only have the two following options:
1. Using a good light and mirror you will have to go inside the door with 10 mm socket (after removing all the nuts and bolts holding the regulator to the door frame) and try to remove the three bolts that hold the motor to the regulator. You will be working in a very tight space and you have to find the bolt heads "by feel." If you can remove the motor successfully - then you will be able to push the window down by hand.
2. If you can not remove the motor then you will have to get a saws all and going into the door and cut the arm off the regulator to push the window down. Of course then you will need a replacement regulator.
This procedure can work on SOME rear windows as well. Does not not on the W201 or W124 models. Those have rear regulators that are run by a cable and the motor is not removable. If you are faced with that type of rear regulator you may have to cut the cable to get the window down.