SLK heater/blower won't shut off
 

This is a strange one! My '99 SLK suddenly started blowing hot air from the vents even though the fan was set to 0. It was a very hot day but I wasn't using the A/C. When I stopped the car the heater continued to blow even with the key out of the ignition. Thinking it was perhaps some kind of extra cooling event due to very high exterior conditions I left it for 15 minutes but the heater was still running. I finally had to remove the 30 amp Temp circuit fuse to get the fan to stop but on the 1 hour drive home I still had hot air coming from the floor vent on the drivers side. I'm pretty handy but can't imagine what kind of fault I'm looking at here and expect the dealer is going to want to change out the climate control system at great expense.
Any thoughts?