Both rear windows on a 95 S420 don't respond to the window button. Additionally the passenger side rear window will shimmy down about a quarter of an inch after a minute or two of driving. I can push it back up to the fully closed position. It's not enough of a fall to see an opening, but is just enough of a fall to hear the sound of the road. The driver side window stays up but is unresponsive to the button. Are these motor or regulator problems?