Yes, this is a 7 year old thread, but I'm doing this repair now. I searched for Sparky's Answers. He has a thread about it.
http://sparkys-answers.com/2010/08/changing-recirculation-fresh-air-door.html
Here is my experience so far in my 2004:
1. Remove dash cover
2. Remove Passenger air bag (2x 10mm bolts in bottom accessible through glove box)
3. Remove Onstar box
4. Remove 10 mm nuts on passenger side securing the plastic dash to the cab
5. remove the u-shape bracket on top of the ducts
6. remove the hose connecting the defrost vent to the mirror vent in the dash
7. remove screw connecting pass. right vent to the dash
8. you should now be able to maneuver the vent assembly back and up giving you access to the actuator and door through the airbag hole
9. the screws on the actuator are 5.5mm, you need a long extension and u-joint
10 best way in was between defrost vent and front vent.
11. I used a flexible magnet with a light on the end to guide the socket and grab the screw before it fell
I have one screw out so far. I will update on replacement as I go.