I am sorry for late reply, get too busy recently.
I am thinking about this too.
My original plan is to physically interface the HVAC control, using servos to turn the knobs, the solution turns out to be too bulky, since I need to preserve the HVAC assembly, mount the servos onto it, and hide them into the dash. Although I am pretty sure this plan would work, there not enough space.
Then the only way to go is to digitally control it, switches can be replaced by relays. For the potentiometer part, the only solution seems to be "Digitally controlled potentiometer", since it is not analog, if build one from scratch, the difficult part is how to program the IC that controls it. A good news is that phidgets seems making something that may be useful in this case.
Above are all the info I have about this, hope that helps.