Vacuum lines under dash that control ac and heating vents,

Look for a vacuum line off or leaking start in the engine compartment than work your way under dash usually in engine compartment but could be under dash usually passenger side or on the back of control switch on dash.

Lots of older cars, especially ones without air conditioning, didn't use vacuum controls, but used cables and levers.
If you do have vacuum lines, will have one from engine's intake manifold going through the firewall and into the back of the control panel to a pod-like device that distributes vacuum to several controllers on the heater/AC case under the dash. Several vacuum lines exit the control panel, each goes to a small diaphragm tank or canister next to the door it operates.

Check to see if the air is coming out of the defrost vents up on top of your dash...by the windshield. If this is the case...you have a vacuum line that has either come loose under the dash....or....the vacuum controller located behind the AC/Heat controller is not functioning properly. ( Tip....Vacuum is what actuates the "doors" inside your AC/Heat plenum...directing airflow straight out front vents...up to the defrost....and down to the heat.....

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Sounds like either the computer that controls the auto climate control has gone out or you could have a vacuum leak. I would definitely check the vacuum leak first. I took my 98 Lesabre to a repair shop several years ago for the same problem. When I picked it up they told me my computer was out and it would cost around $600 to fix. As an alternative, they had unhooked and rerouoted vacuum lines, breaking a tube off the controls under the right side of the dash. They told me that they would have to remove my glove box each spring and fall and reconnect the vacuum lines to change the airflow from floor to dash and vise versa. My fix was to run the lines under the dash just beneath the ash tray and connect shut off valves to the vacuum lines. Now when I need to use the AC I can change the flow of the vacuum by simply turning these two valves so that the flow changes from floor to dash or back. I know this is pretty jack leg but the damage had been done and this has actually worked pretty good.

There is a vacuum leak under the hood. Find the vacuum line on the passenger side where it comes through the firewall and follow it to the engine. You system needs vacuum to operate and without it the default setting is on defrost for safety reasons.

The problem is probably a faulty diverter door actuator or vacuum problem at the HVAC plenum under the dash. This is a common problem - see YouTube for help diagnosing this:
https://www.youtube.com/watch?v=Nlt9A1sp-4U

Check under your hood for broken or loose vacuum lines. Also, the pot on the hood? i believe, on that one. There should be avacuum line running to it. Looks like a little bowling ball. With your engine running , listen for a hissing.

THIS PROBLEM IS CAUSED BY A FAILED PART CALLED A "AIR TEMPERATURE CONTROL BLEND DOOR ACUATOR MOTOR", IT IS LOCTED IN THE AIR DUCT AT THE BLEND DOOR APPROX CENTER OF DASH, IN MOST CASES THE DASH MUST BE REMOVED TO SERVICE THIS MOTOR..

The baffles that direct air, heated or cooled, are controlled by vacuum 'motors.'
You may have a vacuum line off somewhere or the vacuum reservoir is cracked or one of its connectors (thin plastic protrusions) is broken.

Just posted my suspicions elsewhere:

The baffles that direct air, heated or cooled, are controlled by vacuum 'motors.'
You may have a vacuum line off somewhere or the vacuum reservoir is cracked or one of its connectors (thin plastic protrusions) is broken.