Depending on the vehicle the system can operate in several ways.Originally Posted by canadapettit
-The most common is vacuum-assisted moduation where the knobs are vacuum chambers and merely direct a vacuum charge from area to area to determine which way the air travels.
-Electronic systems use a variety of switch and control the flapper doors by small servo/motors.
-The least common way which I dont even know if it is in practice is operational over the I-bus and uses the automatic climate control systems and makes adjustments through the ECU. This would work and is theoretically a great way to further integrate older systems into a new and more elegant system but I dont know of any examples utilizing it.
Now as far as controlling a system via carPC, its possible but it would require being completely custom to each application and would need some little bits and pieces and some coding. I am currently working on the idea with vacuum solenoids and electronic modulation via a serial port. I unfortunately dont have a car fitting to install the system in but as soon as I allocate a 300zx I plan to begin work.