The air distribution drum is a fairly complex bit. It's in two pieces with a single gear system to drive both.
On your ACC system, I'd think the stepper motor would be the prime suspect.
It's very common for the dirverter drum to become quite stiff over time, the usual symptom becomes broken knobs or the white spring loaded link. On the ACC system, the motor replaces those two parts but it still has to fight the stiff drum.
On my '99 SE, I found the brass key at the end of the motor was slipping on the motor shaft. The motor would turn, but the slipping key kept the rest of the system stuck in one place.
To remedy, I followed the "screw" trick to ease the drum and after a bit of careful alignment, I filed a flat onto the motor shaft and crimped the brass key onto the shaft. Has worked like a champ for years.