Sounds like a vacuum problem - leak either under hood or under dash -- could ba a bad vacuum motor under dash Defrost is the default position for this system
defrost is the default mode. in most cases a vacuun leak is the culprit.
You lost the vacuume supply coming from the engine, defrost is the default mode if a mafunction in the vacuume supply is lost.
It is Likely a vacuum leak under dash leading to vents
you need to replace the vacuum.
There is most likely a vacuum leak on the line from the engine to the air conditioner control.
It comes with a heating pad and a thermometer strip, or you can use the defrost setting in your car and set it over the dash vents
google astrosafari van repair and you will find a lot of info on how to repair/replace your vacuum lines and valves
You have a vacuum leak causing your vents to switch from dash to defrost.
Check the vaccuum hoses going to your manifold- The one that controls air flow direction to your AC is probably split Astroman
When there is a problem with the vent system it dfaults to the defrost setting. Sounds like you have a problem in the doors that direct the air flow.
Most likely a bad or disconnected vacuum hose to the defrost/floor selector in the heater ducting.
Vacuum leak