The rear AC in my 2001 Tahoe with manual controls suddenly started blowing hot air a couple of weeks ago. I tried the AC reset procedure, no dice. Then I cracked open the cargo area passenger side panel to investigate, figuring it was either a bad blend door actuator or that the blend door itself was stuck. It turns out that it's neither, sort of. I removed the actuator and was able to turn the blend door by twisting the post (that the actuator attaches to) with my fingertips with no resistance. I noticed that the actuator for the mode door was the same part number, so I rotated the floor/roof vent selector to verify that the actuator worked, and sure enough it did. Then I swapped the two actuators, and the actuator that was originally on the blend door was now working when attached to the mode door. The actuator that was originally on the mode door, and was previously working just fine, would not move the blend door. So, what controls that actuator? In attempting the AC reset procedure, I checked the RR HVAC fuse and it looked fine. Is there some other component that I should look at? I should also point out that I tried both sets of controls - those on the dash and the rear controls in the roof. And as it was, the blend door is stuck in the cold position, so you would think that would be fine for now given that it's hot outside. But the air coming out of the rear vents is just warm, while the front AC unit blows ice cold. Any idea what would be keeping it from getting colder, even with the blend door all the way in the cold position? Bad evaporator core maybe?