89 Caravan heat/air controls

There is a vacuum line running from the engine bay into the cabin through the firewall. This vacuum line will have a check valve on it so that once vacuum is drawn, it will stay that way on the cabin side. The line runs to the back of the heater/ac unit and allows the unit to control the vacuum valves for the air direction. Start by finding the line in the engine bay. It's usually on the driver's side of the vehicle behind the intake and under or near where the heater core lines pass through the firewall. Check this line for cracks or breaks all the way up to the heater control. If none are found, locate the vacuum check valve along this line and make sure it is only allow one direction of air travel and that direction moves air towards the engine. One of these 2 items is almost always the cause of the issue you describe. Post back with results.