Regal air conditioner blows out top vents

Sounds like a vacuum leak. The vents work by using vacuum to open them. When you accelerate the vacuum goes away and the vent door closes into it's resting position. Look under the hood for old brittle vacuum hoses or possibly one got knocked off during maintenance. Another possibility is a vacuum hose knocked off at the climate control maybe during aftermarket radio installation. It could be a bad climate control too because your always moving the lever, over time the control could be worn out and leaking vacuum. Fianlly it could be the actual actuator/piston that moves the vent that could be leaking.
If your lucky maybe you can hear the leak while the car is running, also when you turn off the car and listen (it might not be that loud so do it in a quiet location) for a 'sucking/blowing' if its a big leak it may happen quickly.
For a cheap fix if you can't find the leak you can install an additional vacuum reservoir. It won't make the problem go away but with the additional vacuum storage it will take longer before the vents loose vacuum.
Way back when, car makers used metal HI-C (the juice) cans, now you'll find a black ball about 3-5 inches in diameter as the vacuum reservoir.
Tip: You can use a short piece of hose put up to your ear, block your other ear and move the other end of the hose around the engine compartment to locate leaks or other noises you can't figure where there coming from.