'93 Areostar van blows air only through defrost vents?

I am trying to fix a '93 Ford Areostar van and service manuals are no help with the problem. The AC and heat both work and the fan speed seems to adjust fine but all air comes through the dash defrost vents making the AC useless. This is the longer version van (4.0L) with rear fan controls. The dash is the electronic style, but the Heat/AC switch is standard turn knobs. I have been told it might be a blendor motor or vac. switch but no one, even service techs, seems real sure of this or even where these parts are located.