More rear ac problems with a twist

[Liveforfunjim]

My brand new to me 05 z71 Hoe had no rear ac. Searching around on the inter webs found this awesome resource! I checked my rear actuator for movement when changing from hot to cold and saw no movement at all. I got excited thinking it was that easy of a fix. However when I pulled that off but it came out with a shaft of broken plastic. I'm guessing it's the center of the blend door? How do I get into this rear assembly? What is the part number for the new door? Rock auto has a lot of different options..I'm not sure what I need

 

[Liveforfunjim]

I believe this is called the auto hvac system (a right and left knob up front for temp control, not a slider), with rear heat and ac.