Been there, smelled the burning plastic.  When it happened to me, I knew
there was a problem with the fan pulling too much current (bearings going
bad?), but didn't have time to fix that.  So, I fixed the symptom by
replacing the stock switch with a big ol' high-powered switch from Radio
Shack.  I took apart the old switch, gutted it, and stuck the new switch
through the old hole.  Wire it up and go.
       Someday I will pull the dash and address the fan problem, or try a
fancy relay setup, but for now a big burly new switch is working.  Luckily,
here in Phoenix I don't use the defroster/heater too much.

Bill Marshall
85 GL Tiico