Why not Tangofox?
Did I miss something? I have ruled.out the switch,the fuse is good, the only thing left as far as I can tell is to supply 12v to the wiring at the switch then to the motor,
If it works with power supplied directly it would be the wiring from the switch to the motor. But I can't believe the wiring fails with no warning. I have seen old wiring fail but never without a warning of sometype.
I am an electrician by trade is how I arrived at those conclusions.

If I mikssed something please tell me rather than post a negative comment without backing it up.

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86 300SDL. 250,xxx on #14 Head. One eye always on temp gauge.. Cruising towards 300K