IT is possible, that it broke, and is on an ANGLE, and has not fallen down into
the box.
REMOVE the actuator (NOT SOLENOID), it is on the driver side. When you remove it,
you will know immediately, if the blend door broke. You will not see the end
with the "+" grooves in it. Also, if you remove the actuator, and you hear
a PLOP, then it fell the rest of the way down, because it is broken.
It is a problem that GM has not been able to fix. When you use the knob to go all
the way cold, the door stops, but the motor will keep torquing on the door, and
keep torque on it, and the end twists off, and eventually breaks:
You have to stop before MAX to prevent that. Eventually, the end gets twisted off the door, and it falls.
You can take the screws out of the actuator, and turn your knob, and see
where the motor starts to move, back off a tad, and look at where the
knob is. THAT is where you stop for going cold.
GM's solution is an updated part, that is reinforced to help prevent it from
happening. I mean, the cost to replace it is enormous, so they better
had replaced it with a part that will withstand the punishment.