No air coming out of rear vents

Shane, Base 235 75R16, SLE, SLT, XL SLE, XL SLT P245 65R17,
Denali, XL Denali P245 60R18. Check the attached links,instruction and guides, Good luck
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GMC Envoy Tire Sizes
2006 GMC Envoy Tire Sizes
Tires for my Envoy Denali Recomendations

Assuming you're talking about the HVAC mode actuators, below are a couple of diagrams showing the location of components in both the front and rear HVAC units.

The blend door or its actuator in the rear HVAC unit (behind panel on passenger side cargo area) is either stuck or defective.

I have the same car and the same problem.......

the heat accuator could be bad. It is a motorized part that adjusts your temprature by allowing cold or warm air flow when you adjust the temprature at the knob you are turning

{: ) The popping sound you heard is caused by the sudden movement of an HVAC door, most probably coming from the recirculation door actuator (right dashboard area) or the mode actuator( mid dashboard area). The fan outputs could be normally working, but is being muffled by the incorrect positioning of the HVAC doors, giving an impression that there is fault in the blower controls. In other words, your vehicle is having an air distribution issue.
If there's no conflict in your climate settings, then the trouble may be in the HVAC doors binding or broken, or the actuators faulty or sticking. In both cases, servicing may involve removal of the dashboard to access these HVAC components.

on the end of the center pin there should be a screw coming in from the passenger side of the diff. that needs to come out first.

Just behind right rear tire up in the wheel well. Air inlet is inside right rear quarter near the accessory inflator. Compressor runs 3 lines one for the accessory inflator and one for each air bag.
THere are also 2 sensors, one on each trailing arm that senses load and then raises and lowers accordingly.

sounds like you have two problems, if there is no air from front then you need to ck power to your blower motor, if no power you need to ck your blower resistor and connector, second if no cool air from rear your compressor may not be engaging, make sure you ck your fuse for the front hvac system.

The fuse sometimes located by rear controls or could be switch faulty..Double check that!