I have a 96 Ford f150 Flareside. When I turn on the heat/AC it’s blowing but doesn’t come out the vents.
Where the air comes out is controlled by vacuum servos. If there is a major leak or if the vacuum line is no longer connected to the vacuum reservoir, then the default is for all air flow to come out of the defrost ports at the base of the windshield.
Sometimes, with P/U trucks, you can remove the glove box and then be able to see the behind the dash heater controls, a system of vacuum servos and damper doors that direct the airflow.