Only blows out defrost

It's the vent actuator, it moves the baffle that changes the direction of the air flow.
Here is a video of a broken vacuum hose that creates the same problem.

It could also be a failed actuator.

sounds like a big vacuum leak, or a bad check valve for the vacuum system. look at decal for engine under hood and trace out the vacuum system

check for a broken /loose vacuum line on the engine . Trace it back to the fire wall and it will most likely be the culprit/

Check the vacuum lines in the engine compartment that go to the firewall if your A/C switches to defrost on your 1997 Cadillac Deville. Find an approx 1/4" rubber hose that goes into the firewall around the center of the firewall and trace it back to the motor and make sure it hasn't come loose. If not that, then it may have come loose at the a/c control panel. hope it can help you and good luck---------------------------------------------------------------------------------------------
OBD2 Fault Code Reader

Sounds like a broken vacuum line..When have no vacuum it defaults to defrost...Check outside by the firewall on the passenger side..There should be a black and gray vacuum lines..Trace each one down and look for one of them broken or unplugged..They are notorious for that...

floor/defrost is default mode when system looses vacuum.you need to trace lines from the
intake manifold back to mode selector(floor,vent,defrost)in car for vacuum.usually worn crack
vacuum hose

You need to trace the vacuum lines that run from the air box at the fire wall to see which one is broke. Once you locate that buy a piece of rubber hose that fits tight and slide onto it. Should fix the problem.

You need to check for a vacuum leak. You can use WD-40 to spray along the vacuum lines in the engine compartment, use caution it is flammable so only spray a small amount at a time. Trace all liines, caps, and resivoirs for vacuum and if they are sprayed with WD-40 or Carb. Cleaner, the engine will slow or stall when it sucks in the fumes.

Fix any leaks and this should fix the problem. You may also need to check for any vacuum lines inside the dash area going to the a/c diverter, if it leaks, the default could be to stay at defrost.

Some vehicles have a small tank to store vacuum that supplies the a/c system with a constant supply, these can be in the engine compartment, usually black & round, but also are located under the fenders, hidden. You can trace your vacuum lines to identify if you have these or not. Even similar models can be different so tracing is the only way to really know.

good luck

On the passenger side front to the fire wall above the blower fan the lines run to there!! Trace an check for leaks an cracks!!

You have a vacuum leak,defrost is the default postion(no vacuum).Look under the hood on the passenger side at the firewall and find the small black vacuum line that goes inside the car.Trace this line back, you will find it goes to a black canister(which is the reserve supply) then it will leave this and go to a vacuum port on the intake manifold.You will most likely find a leak somewhere in theis line.