[randman1]

The airflow is directed to the defrost vents by default in the event of a vacuum loss. Usually I would tell someone in your position to check the vacuum line going to the reservoir (vacuum tank) under the hood but I don't think you have one there. The vacuum source for the climate control may come from the brake booster. What you need to do is inspect the vacuum lines for cracked, brittle, broken or disconnected lines or connectors. I believe there is one that runs along the firewall from the grommet behind the passenger's side shock tower towards the brake booster.