vent select knob probably means its a non auto climate control head so im gonna say its a vacuum actuator

check the vac supply line/resevoir bulb for vac, the line should go across from the pass strut tower to the 3 way vac tee on top of the blower

ive seen the knob break but thats usually after the initial problem as they "beat the **** out of it" trying to get it to work