As per advice from one of the techs here, I located the black box thing behind the right headlight under the hood. It sits on top of a junction of 3-4 vacuum hoses. On top it has a rectangular device with numerous wires plugged into it. Is this the vacuum pump that opens and closes the vents and lets heat/cold air into the cabin? Can it be easily removed, tested and replaced/repaired if necessary? As I mentioned in an earlier post, my blower motor runs strong, the system blows hot air on DEF. only, with no activity on hi-auto, low-auto, or bi-level, and no air conditioning, although I can hear a switch go on and off under the hood when I turn the air button to "on-off".
I hope this isn't TOO confusing.