I have a 2006 tahoe that does not have any rear heat. I saw a post on a 03 tahoe with the same problem. Mine has the auto climate control and was just wondering if the same procedure
applies for the 06 as it did for the 03. I have not checked to see if the heater pipes are warm to the back. Thanks,
old tech

 

I had the 03. It took about 10 minutes to get the panels off and unscrew the actuators. Once you have them off, you can turn them manually to verify if that is what the problem is. I have one ordered to replace the bad one.

 

It is the same proceedure for 2000 - 2006 model years.
If you find that the actuator/door is binding order GM part 88891577 to replace the bad assembly.