1994 Infiniti J30 air condition

If the air blows only from the defroster vents, whether using the heater or air conditioning, and regardless of which mode is selected, you most likely have vacuum-operated mode doors, and a break in the vacuum hose. On most cars the mode doors are spring-loaded to default to the defrost position for safety reasons. Your feet might freeze but the windshield will be clear.

There should be a vacuum hose routing diagram under the hood, on a sticker. Look on that for a small hose and check valve going through the firewall. Check if that hose is cracked or disconnected.