BMW 730i E32 windows

[Erich]

my friend,

under the rear seat there are hundreds of cables. I know it because I just disassemble a car. If you cannot read wiring diagrams, then you are lost. Wiring diagrams are on my website.
But first of all do the following:
-remove the window switches in the center console and clean them. Often some drink spills into that area and the switches are not having contact.
-check on the rear windows the switches at the doors. Do they work?

If they work, then I would exclude the GM and RM under the rear seat.
But you can remove the GM and the RM and spray the contacts on them.

Broken wires in doors: usually where they go from the A-pillar into the door.
Check the drivers door first as this is most often used. Open the door and check if the windows are working in different positions of the door open.
See here