try this again, I have a 1998 Ford Mustang, when I turn on the a/c, air only blows out the defrost vent, no matter what setting it is on, how can I fix this?
It sounds like you have lost the vacuum to the vent door control. The defrost vent is the fail safe mode.
The vacuum hoses are usually rubber. Either a hose has gotten hard and fallen off one of the vacuum connections, or (less likely) a diaphragm has failed (hole or frozen up). Check under the hood while the engine is running and see if any rubber lines have fallen off, then check under the dash. You should be able to hear the hissing of the vacuum leak to guide you inside the car.
I couldn’t hear a hissing on my vehicle but I traced a small diameter black plastic hose from the valve cover across the engine to the firewall. The heat fron the engine causes the plastic to get brittle and crack. You can replace the plastic hose with regular vacuum hose from an auto parts store.
That is definitely the HVAC system hose. I had the plastic hose melt once. Had to cut it and splice in rubber.