Next time ask the forum before buying a switch, if you got it at West, take it back for a refund!
There's a gear head at the base of the wiper arm shaft, in there are long contacts that continue the power until the arm reaches the rest point then disconnects the power, the contact appear to have malfunctioned, perhaps SW got in there. If you are a DIYer you may be apply to take it apart and fix it, and reseal the water entry point, or just replace the whole thing. The motor has three connections, a neg feed, a pos feed from switch, and an always on pos feed.
Disconnecting the always on power feed or neg feed - stops it cold when it goes postal.