I don’t have an answer but how is the “S” terminal wired?

My dad was into boats in a big way. He had several older ones over the years. They were all 12 volt DC. Fwiw, your comment implies reasoning about why his boats always had 1 switch per wiper motor. He didn’t install them but it was a consistent feature over 5 boats that had wipers.

I’d guess that you’d run a positive wire to each motor with a fuse on it and share a ground. Maybe the "S" terminal intended as a ground between the motors?

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