1989 560SEL, 96k miles. Windshield wipers (ww's) occasionally will not work, like when it's raining. I'll switch the ww's on, they will work for a while, then freeze. They will not move no matter what position the switch is in. Or, they will be working, I'll turn them off for a while, when I try to turn them back on they will not work. If they are not working, I park my car, come back to it later, and the ww's start working again. Very Intermittent. Where to start, where to start? Thanks, Scott