I have a 1989 190e, different, but related, so my experience may be relevant. The window wires break inside the pillar betwen the front and rear doors, just before they enter the rubber tube to the door. I don't know why. You would not think that there would be much flexing. Both sides had the same problem.

You can check for continuity between the front console switch and the rear door plug, if you have a wiring diagram. On the 190e, the wires have the same colour code at each end, so it is easy. Probably yours is the same. One is always hot (brown) as long as the front console switch is plugged in.

On the 190e, you remove the panels on the inside of the pillar, exposing a hole at the bottom. You can see the wires going up the pillar. You have to disconnect the seatbelt. The rubber tube is big enough that you can leave it, and work from the door, for the other end of the wire. The broken wires (I always found two, although you would only expect one) will almost fall out on their own. It is easy to solder in an extra length to bridge the gap, and cover it up with shrink tubing.

Good luck!

Andrew