Problem with defrost/vent

Hello,
My heat and AC work fine, however recently after hearing some odd clicking and shutters. Now I only get warm or cold air blowing out of the side vents and floor, none from the center vents and more importantly, none from my defrost setting, which makes it really hard with winter coming on.
My question is, which actuator is out and where would it be located on my car?
Any information you guys can provide would be greatly appreciated.
Thank you ahead of time!