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The switch provides the ground for the different speeds. It has to be mounted in the dash before the wipers will work. There is also a ground strap on one of the wiper mounting bolt holes. It is integrated into the rubber bushing. Make sure it is there.
Here is a thread that might help you figure it out....
http://67-72chevytrucks.com/vboard/s...d.php?t=413551
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