Since the humidity here rivals that of living under water, I had to clean my window switches on my ex-300TD (1981) every year. As stated before, just open the switch very carefully and clean it throughly inside (I used a cotton swab soaked with the aformentioned RadioShack spray) Did it every time, the windows would start working perfectly again!

A. Rosich
S320, 1998
E320T, 1995