Easy. The wiper transmission is worn out. Time to replace.....

Or..

Take the torq bolt off the top of the wiper arm transmission. pull off the mechanism. You'll see a raised edge that the mech is supposed to catch on. Since its so worn down the mechanism passes over it when it spins backwards (when you turn the wipers off) you have to bend it up.. put it back together test it to see if you bent it enough.. I've fixed a few that way.

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