I'm not familiar with that particular model, but it sounds like a vacuum problem.
All of the actuators for air direction are normally contrilled by vacuum motors and
the system will default to defrost (demist) if the system loses vacuum. Look closely
at the vacuum hoses on the firewall in the engine compartment for breaks, cracks, etc.
There will be one line in particular that feeds the climate control. It will go
through the firewall to the passenger compartment and often has a plastic check valve
in the line somewhere. I hope this gets you started in the right direction.