I agree with Marti,
If you only have the problem on one window you could test the theory by swapping the switches over. (Lift your shifter gator and push them up from beneath, unplug them and swap them). If the problem moves to the other window, then you know for sure. You can strip them down and clean the contacts, but be careful. You might as well do them both for good measure.
97 hell red 328i, M3 front, Eibach springs, 18" dished Alutec schnitzer typeII replicas.