I can't switch the HVAC from the defrost, to the panel, to the floor, etc. It's stuck coming out of the defrost (which is the default setting, I believe). I've checked the head unit itself and all the vacuum lines appear to be in order. Anyone have any other suggestions I might try?

thanks in advance...

 

d'Oh! I forgot to mention, it's an '02 quad cab 4x4 dakota...