The air distrubution system on your vehicle is vacuum controlled. Start by inspecting the vacuum hose and check valve in the engine compartment. Install a vacuum pump and see if the system can hold vacuum. If that is good, then the problem most likely lies inside the dash. A series of tests must be performed at the vacuum switch. It could also be a failed actuator. The technical degree of difficulty increases at this point, so if you are not comfortable with under dash repairs, a qualified technician should be involved.