'95 525i Windshield Wiper Problem - need help

My '95 525i has a strange problem with the windshield wiper system. If I turn on the wipers and leave them on for, say 5 minutes, then move the wiper control to either the OFF or the INTERMITTANT position, the wipers keep going. Even if I turn off the ignition switch and then start the car again, they will (sometimes) remain on. To turn them off, I move the wiper control to OFF and then just wait - they eventually stop after a few minutes. (sometimes when they stop they stop mid-cycle, too).

I'd really appreciate any help on this. I thought that maybe it was the switch on the column but I'm not convinced that this is where the problem is.

Thanks.