I figured this out for my '84 300 wagon and got my right front and rear windows working again... for about 4 weeks. They gradually took more and more tries to get the windows back up. The day the front drivers window didn't budge after 3-4 minutes of praying and button pushing I had to do it all over again in the parking lot at the correctional facility where I work. They don't let us leave our cars open when unattended! I just did it again after about 6-7 weeks - at work again; why do today what you can put off for tomorrow? Each time the same posts were crudded up.
The problem seems to be that the contact posts inside the switch are worn down from 23 years of use and get a build up of corrosion eventually blocking current. Electrolitic grease didn't stop this. I'm considering building up the posts w/ solder, theory being that the slight gap is causing sparking which creates carbon residue thus blocking the current when too thick.