Okay, the mention of corroded brings another thought? If the connector looks like it is corroded. it may be the cause! Maybe take a minute to scrape the contacts if they look bad. That can be as simple as running a screwdriver point up and down over them if they are a blade that sticks out or if a person had an emery board like for nails? Not to get too agressive and file off too much but just to make them shiny.
Sometimes something like WD-40 or contact cleaner are good, too.
The idea is pretty simple like this drawing. the switch puts power on different wiresso that it hits the route to the motro at different places that give more or less resistance. You get different amounts of power getting to the motor and different speeds. But if the path is open at the red mark, you get nothing on any speed!
Good luck on the chase. We never really know we found the problem until it starts working.
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Richard
Why no RV year, make and floorplan on MY signature as we suggest for others?
I currently DO NOT have one!