Heat, Vent and Cooling at dash, full heat out of rear heat ducts

[glmoore0001]

Hello.. looking for a little advice here. 2006 Tahoe Z71. Whether I am running the vent or the ac (which works fine at the dash) I have full heat coming from the headliner vents above the rear seats as if this heat was completely bypassing the thermostats. even with the temp set on 60 at the dual climate control, it seems that nothing I do on the Climate Control panel affects the rear heat vents.
Any ideas for me? It is like there is a stuck blend door or something, but I would think that even a blend door would be at the same temp as the dash.
I did a search on the subject but came up with no results.
I am a decent mechanic and do my own work. I appreciate any direction you can give me.
Thanks.. Gary

 

[Scottydoggs]

theres a blend door actuator in the rear pass side hvac unit. and theres also a module that goes bad some times as well mounted right next to it.

heres a vid how to open it all up, this one he changes the module, you can see the blend door actuator as well.

 

[glmoore0001]

Thank you, that gave me what I needed. Between that video and this video

I now understand the working of the rear heat. Thank you much for the help! Gary

 

[Rocket Man]

I just replaced a rear actuator that fixed this problem.