I had this on my E34 Touring recently. I lubricated the all of the window
guide rubbers with silicone oil (Pledge often works too!), and took the door
cards off and regreased all of the linkages and operating arms.
The rear windows are now better than the fronts!
Don't let them get any worse or you'll find the torque from the motor breaks
the nylon guide 'pegs' in the 'scissor' part of the operating mechanism.
This is a bastard to fix as the parts are not available separately. (I got
my 'spares' from a local breakers but still had to strip out the doors on 4
cars to find a good pair of pegs!).
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