Slowly been playing with this, and the prototype is basically done. I may have discovered that the servos are not powerful enough and my new math has discovered that the servo horns need to be longer. IIRC these servos are all plastic, and rated at 96 inch-oz (.5 ft-lbs). I thing with all the crap I bought and wasted/redid I'm in it roughly $200. Half of that was the STAMP board which could be replaced with an Arduino for much cheaper but I wanted the future expandability. Here is what I've come up with so far...
This is the controller/servo box, power and resistance value from knob in, two cables out. The power will be a switched 12vdc-6vdc converter located under the dash somewhere. The box is roughly 6"x4"x2.5", I'll most likely velcro it to the carpet behind the center console.
This plate and switch replaces the gear driven arm and cable assembly from the back of the climate control. Factory screw holes and locating holes, plus its much lighter than the stock stuff. The factory knob should be able to be retained with some modification and a small set screw. Three wires to/from the potentiometer; ground, +5vdc, and a return voltage. Pretty simple.
Everything inside of the project box - basically everything was purchased from Fry's. Home Depot had the hardware. The STAMP board has lots of room for added whatever, and coding is easy to update and test. I am using 1 I/O pin and 2 servo headers, so easily 10 I/O pins for whatever in the future. The breadboard looks like crap, I would like to move everything up top when it is up and running.
A lower shot of the servo/cable board. Not quite perfect, but it'll do. Spaced the servos down a bit, then the board up a bunch. Fairly rigid. Note the 4 pin 90 degree molex header... three pins will be soldered/wired to the potentiometer once the length has been determined, completing the RCTime circuit on the breadboard.