The control head for the mode selector is operated by vacuum. Start in one of two places.
1) Under the hood a black vacuum line about 1/4" runs into the passenger side fire wall. This is the vacuum supply to the heating ventilation and air conditioning (hvac) control head. Disconnect it at one of the rubber links and check for vacuum while the engine is running. If it is good then you will need to go to step 2. If vacuum is missing, back trace it until you find a pinched or broken connection.
2) Remove the control head the hvac unit. On the back side are a multitude of colored 1/4" hoses leading into the mode selector switch. Carefully disconnect that assembly. Start the vehicle and check for a sucking with your fingers. Likely if it was not found under the hood it will be missing here. Back trace the supply line from here to a pinched or cut line.