BMW: crazy switches...

I have a problem with the windshield wiper switch that projects from the
right side of the steering column (1990 325iC). Basically it doesn't work,
or rarely at best...BUT, if I toggle the handle beneath it (cruise control
I believe) the wipers miraculously work(!).

Clearly, some wires are crossed
somewhere...or is this explained in some footnote of the manual (that I
never got with the car)?

Thanks, A.