The VW wiper motor uses dynamic braking. Essentially, when you turn the wipers Off, they stop when they reach their park position, but they also get retarded electrically to bring the motor to a rapid stop. When you turn the wiper switch Off and the wipers reach the park position the armature of the wiper motor is disconnected from power and immediately connected across ground. The motor then functions as a loaded generator and develops a retarding torque that rapidly stops the motor.
There's a good write-up at Speedy Jim's site: http://www.nls.net/mp/volks/schem/wiper2.gif
In the stock wiring, the solid black wire goes to Term 53 (low speed) and the black/yellow-stripe wire goes to 53b (high speed).
The black/purple-stripe wire goes to 53a, which provides a source of power via contacts in the transmission to the low speed windings when the switch is turned Off until the blades reach their rest position, when those contacts open and another set ground the slow speed windings to stall the motor.
It appears that your generic switch is not fully compatible with dynamic braking, since it lacks the switched ground (Term 31b) found on the stock switch (refer to Speedy Jim's diagram). If you were to provide an un
switched ground to Term 31b on the wiper motor, Low speed would be useless since every time the blades got to the rest position the contacts in the transmission would create a dead short from ground to Term 53 and blow the fuse. The switch may
work if you simply wire Low to 53, High to 53b, and Park to 53a...but don't be surprised if the wipers "coast through" the rest position (at least sometimes) making it impossible to shut them off without reaching out the window and manually stalling them. If you left Term 53 disconnected you'd have wipers that always parked reliably, but you'd only have one speed - High.The below refers using a Cole-Hersee electronic intermittent switch on VWs; nothing to do with your question, I'm just posting it here for the benefit of anyone searching for that subject in the future.
The Cole-Hersee electronic switch also lacks a Term 31b wire, so it would appear to be subject to the same problem mentioned above - but it works perfectly, because a microprocessor inside monitors the current draw through the Term 53a wire and immediately grounds Term 53 when the transmission contacts open. This provides dynamic braking with no wire connected to Term 31b.
Here's how to wire that switch to a `67-`69 style VW wiper motor:
To a `70/`71 (electrically identical to `67-`69, just looks different):
And lastly, to a `72-`79 motor...the park circuit is different on cars with the steering-column-mounted stock switch, but wired like this it'll work: