[wiswind]

OK....my '96 did NOT have the rear heat/AC unit, so I have NO personal experience with that.

The FRONT unit issue that you are having is a lack of vaccum to the actuators that hold the diverter doors in position.
With NO vaccum, they will be in the position to direct all air to the defrost vents (windshield).
If you look up under the dashboard....you will see the door actuator "bulbs" with the vaccum lines running to them.

There is a vaccum "resevior" in the engine compartment.....on the passenger side firewall.
This box holds a vaccum when the engine is not running, to hold the doors in position until you start the engine again.
I would start at that location.....and follow the vaccum lines to see if you can find the location that the vaccum is lost.
It can be lost due to a disconnected vaccum line, or broken or smashed vaccum line.

Do not confuse the diverter doors for airflow with the "blend door" that controlls the mix of hot/cold air to control the air TEMPERATURE.