Unplug that switch and see if the problem goes away . . . Double check the wiring where is comes through the body and up to the defogger. If unplugging the switch doesn't do it check to see if there is a fuse just for it in the fuse panel. Remove the fuse. If that cures it there was a short in the wiring for defogger.
Shorts can drive even a wooden man crazy . . .

I've been known to stand in the corner talking to myself . . . and I usually had a wiring diagram and a test light to run shorts . . .
Remember: If a wire is burnt . . . It will only burn from where the short is to the source of the power . . .
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Cheryl . . . Mom, mechanic, and gopher
1, 8, 4, 3, 6, 5, 7, 2 What do you mean I can't use that on a FORD?