'When function door is in defrost position, air inlet door vacuum control motor is at no vacuum, admitting only outside air into passenger compartment. Panel/floor door and panel/defrost vacuum control motors are at no vacuum, directing air airflow to defrost duct and side window demisters. Temperature can be adjusted to heat or cool air below outside temperature. A/C compressor will operate if outside temperature is above approximately 43*F (6*C). Blower motors is On.'

You're either not getting a vacuum source to the Panel/Defrost Vacuum Actuator or the actuator's diaphragm has a hole in it.