Are you talking about the vent at the base of the windshield??? If so that has been there since the Mustang came out. It originally supplied air for a left/right floor vent. Then when A/C became built in, 1968, it also supplied the outside air to the evaporator case.
It has two drains, left and right, that are under the fenders. They clog very easy with leaves and debri. Then "RUST" comes and never sleeps, till it has eaten a hole into the passenger compartment under the dash and leaks like a screen door in a submarine. Then the now wet floor "rusts" under the carpeting.
"Rust never sleeps"
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