Velleman make an intermittent wiper control. I have fitted one to both my B12 and Flavia 2000 coupe.
The circuit board is about two-thirds the size of a credit card and is easily mounted behind the fashboard and using the spare hole next to the three existing knobs. The device provides a single wipe at, as I recall, every 5, 10 or 15 seconds.
Alternatively, if you just want the wipers to move slowly experiment with a low resistance in series with the motor. I think something like 5 ohms or so. Needs ro be a ceramic, wire-wound type though.