1993 Buick Roadmaster Buick Roadmaster Wiper problem

I have a 93 Buick Roadmaster Wagon Esatate.

My wipers work on continuos standard speed 1 and 2.
They do not work (no movement at all) on any intermittent speeds between slow and fast.

All else works- constant speeds, 1 wipe swing and washer fluid shoots ok.

Failure is only with intermittent speeds.

Thanks

sfjose