a/c not blowing out the main vent
Your heater/ac register controls are vacuum actuated. I would start under the hood and locate the vacuum supply coming from the firewall (right side above or near the black plastic heater case) and trace it to a "T". There will be a vacuum line to the intake manifold (supply), and a vacuum line to a vacuum canister or reservoir. There is also a check valve that allows the reservoir to take over in case of a low vacuum signal (like passing or pulling a hill). I work on Crown Victoria Police Interceptors (same car, different badge, more fun). Usually it's a vacuum supply concern - a broken line.
Ac not coming from ac vents
You could try to see if the solenoids are actuating the control the flappers that change the direction of air flow there's a couple of them. They work off of the vacume system.
Most common cause is a vacuum check valve gone bad --located under the hood passenger side this is a unit about $70.00 can be easily changed with just nut drivers. (this is on a 2003 Mer. Grand Marquis.
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