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Did you check at the plug or is that what you meant? For example...did you remove the plug and checked the contacts? I don't mean to make you sound like a dolt, because that's probably what you did, I just want to get the obvious out of the way. Yours may be more complex but I was able to open mine up and clean the contacts.
Yup...happened to me on my old mercury I had in high school....power windo motor quit working, and after we disconnected and cleaned the contacts, and reconnected, it worked fine.