I have a similar problem, '90 C2 Targa: when I got the car a year ago both heater and AC worked fine, then recently the fan went crazy: if I leave it at 0 or 1 it would go all the way to 4 on its own (with the knob still at 0 or 1) and the way to turn it off was to put the knob at 4 then rotate it back to 0. It did this for a couple of months, then stopped working altogether.
With what you're saying, I bet that if I jump out the fan everything will just work. So I'm also hoping someone will help here!
