Heating system problem

Now that the snow has started to fall I noticed my windshield defroster will only blow on the passenger side of the dash. I do get alot of air also blowing at the side windows. I hear a whistling sound from the dash. Im assuming that the duct to the windshield must have 2 vacuum actuated flaps and the driver side is not moving. Would that sound correct? What would be the easiest way to get in there to repair? I am thinking that i may have to remove the entire dash. All the other vents do work when selected. Thanks