E30 Windshield Wiper Motor/Switch?

My 1989 E30 325i wipers operate only at the fastest position. If I try
to use any slower speed or the intermittent setting, most of the time
wipers don't work. What has got me puzzled is that I can start the
wipers (as in washing the windhsield) and all of the motor/switch speeds
work fine. I have checked the fuses and they are OK. I tried repairing
the switch by removing it from the steering column and spraying WD40 to
clean possible dirty contacts inside the switch. Didn't solve the
problem. If I was sure that the switch was bad I would replace it, but
it does work. Any ideas of cause and cure would be appreciated.

Thanks,
Patrick