Ihave a 1992 caddy deville Ican hear the blower motor blowing .

Sounds like high blower relay is bad

I had a 2003 caddy dts same issue the blower motor has probably gone bad u can order them online. What u can also try is kicking under the passenger side glove box area that's where the blower is located sometimes they get stuck. Try kicking or tapping under there and see if it pops on. I did this with my car

More than likely it's blower motor of blower relay. GMs are notorious for blower motor failures because they're running constantly when engine is on. (If you'll note there is NO off position just low to hi)

The blower motor is either bad, or if you hear it blowing in the front but nothing comes out of the vents, then it's a vacuum motor problem. The flaps aren't angled right to allow air to come out of the vents (either for a/c, heat, or defrost).

The 1992 was prior to Buick putting cabin air filters in so there is not a possibility it is clogged. So more than likely your blower motor is going bad or your fan control switch is bad. Does this problem occur with heating as well as AC? Does it change if you switch between blowing on floor, face, window(defrost) If it doesnt change based on location of blow and type of air temp then I would start by turning fan on high and placing head (ear ) against dash in center of console area. Have someone turn fan knob from low to high with engine OFF...After goign from low to high and back a few times you can see if the blower motor is responding to signal from switch. Listen for a whine, squeal, rattle, or humm. A whine or Humm would indicate a bad blower motor... Squeal or rattle a bad squirrel cage/fan blower blades. If you hear no noise and no perceivable change in blower speed/noise then its possible your switch is bad. Unfortunately the only way to definitely diagnose either is to pull the switch from dash and use multimeter to test for voltage change and for blower motor to pull dash. There is a digital control unit for your car as well. Although rare sometimes the wires on this can loosen or it can go bad. But since you didnt state teh fan wasnt working at all and you are getting cool air, that woudl be teh last thing to check

could be but caddy's have 2-3 heat and air computers on there cars that control diff functions of the system seperately either one of them can be and will be bad !! they control the vents to temp control valve and blower i owned enough caddy's to know this!! this is why!! i drive a chev truck today LOL

Have you tried smacking the blower motor? If it starts running after you hit it, replace it.

is the fan motor working at full speed
check for blocked cabin air filter
the vacuum only works the vent controls and as you can feel the changes of the vents , it is not vacuum

For starters, do you hear the blower motor running when you turn the heat on? If it does blow is there any airflow coming from your air vents? If the blower motor runs and there is air flow from the vents there are a few other tiems to check. Veriiffy that the coolant is full, low coolant level will effect the flow of coolant to the heater core. If coolant is full you may have a clogged heater core, or one of the blend doors ( they regulate air flow between the A/C evaperator and your heater core) may be malfunctioning or the control head(temp control) is malfntioning.

There's a blower motor (fan) under the hood and up against the firewall near the center of the car that takes some work to remove. It has gone out in the last 2 of my 4 Cadillacs. Turns out if you're not real careful with the positioning of the spark plug wires they will brush up against it, and if one of them gets cracked or brittle it will "arc" and short out the blower motor. The a/c compressor should still come on (you'll hear a click when it engages) but you won't get any air blowing into the cabin, hot or cold.

The part was like $200 stock (year 2000 prices) and it took about an hour to remove it. My Caddy at that time had this damn strut bar going across the engine bay which we eventually removed to get to the blower motor.