This issue is for a 2012 GMC Acadia Denali with the digital driver and passenger side separate temp controls. The initial problem was the air would only flow through the defrost vent. I researched and found the suggestions to change the blend door actuator. When I was searching for the replacement, I found the Doorman models 604140 and 604141. I was told 604140 is for the front of the vehicle, while the other is for the rear. I ordered the 604140 model. https://www.amazon.com/gp/product/B00HJ6Q9RG/ref=oh_aui_detailpage_o03_s01?ie=UTF8&psc=1

The problem is I replaced the actuator that is by far the most difficult to reach, which is also said to control the dash/defrost/etc. The dial on the back of the actuator I removed was in the position farthest to the left (on a clock, it would be the 10 o clock position). That position must be the defrost. The new actuator was in the middle (12 o clock position). Neither the old actuator or the replacement would turn by hand when I placed the plastic wheel in the slot of the actuator. I didn’t know if this thing is supposed to be able to turn on its own or if it has to have power. I opened the old actuator. I didn’t see any cracks or anything. Everything still looked new.

Now that the new actuator is in, the air now blows weakly through the dash. I can’t move it out of this position. It’s probably going to other places but it’s too weak to say for sure. When I replaced the actuator, then reconnected the battery, I inserted the key without starting the engine. When the power was on, I heard the grinding of the gears that was described in the article. It lasts about 5 seconds then you won’t hear it again. Before the replacement, on occasion I would hear very light slow clicking. It was nothing like this. I’m wondering if the plastic wheel needs to be replaced. When I looked at the wheel, it didn’t look stripped. By all accounts, this should be fixed right now. I’ve worked on some other things like the headlights. Whenever I reconnect the battery I hear that fast, loud grinding of the gears that I never heard previously.
Speaking on behalf of everyone who has attempted this, being from Detroit, if I ever run into the engineer who designed the location of that actuator or fuse box, we will have a very interesting conversation.