94 Mustang GT A/C

Especially that little thin plastic one.
From this thread: https://www.stangnet.com/mustang-forums/threads/air-only-comes-out-of-defroster-vents.783650/

Open the hood and find where your A/C hoses come out of the firewall which should be on the passenger side. They are metal pipes and there should be a small black vacuum hose close to them. Follow that from the firewall to the connection in the engine compartment. You'll either see a break in the hose or a pinch.