I think I may do the same sort of install except onto the outside of my sled. It would be easy to pull in cold air from the ac to pump into the unit due to the position of the ducts in my '98 M3. The sled-fan would line up with the empeg holes in the case, and I'd mount the thermister in the dash below/behind the empeg (away from cold air vent). I'll build the switch into my dash somewhere in case I ever use the heater.

John