I have a 1994 Cadillac Fleetwood. I just had the thermostat replaced, had the a/c diagnostic because there was only warm air blowing out and I had the air filter replaced. After this i noticed that the heat will only blow from the defrost and not out of the vents. this all happened after the car was serviced. Please adice on what the problem could be.

Sounds like some of the air duct work was reinstalled backwards or the cable which opens and closes the duct flow was not replaced properly. I'd go back to whomever made the repairs and have them fix it

Somewhere along the line one of the vacumn feed pipes to the heater unit has been pulled off or squashed or something stupid ,now best one here is iam in europe so i have never seen one to be able to tell you what pipe is where so go back to garage that serviced the vehicle and ask very nicely if they could have a look for you in case the fitter has missed something and i guarantee they will bend over backwards to help you and do it for free and give you a loan car till its done ,print this reply off and show it to them and ask nicely ,somebody start shouting at me i just tell them where to go but ask nicely and i will always put any mistakes right for free weither they were my doing or not ,but i do have a good customer base and i dont advertise

Hi!!

Check HVAC IGN 10A Fuse in the passenger compartment fuse box. If fuse is OK, check the flow control door, located on top of evap case. remove glove box look way up high to the left, it may be stuck. Turn the ignition switch ON, turn the air on and activate the flow divert control while you look if it works, activate the door manually to get it loose if stuck. if no movement is detected, the control may be defective and needs replacement.
Keep me posted

The heated air is taken to the vents by some pipes. These pipes are controlled by a flap, the flap itself is moved by the lever that is controlled thru a cable by the button on the dash.
In your case the flap is such. You must open the dash and find out why - the flap itself has been sized in place by dirt, the lever is broken, the cable is broken or detached , or the button is faulty. In the end all you need to do is to clean the air pipes and replace the broken part.
The fact this happened after a service is just a coincidence.

Are you absolutely POSITIVE that BOTH cooling fans are operating during this fault period?

When it is extremly hot and or humid this is considered normal provided you have tested the freon pressures.

You will neeed to put aset of AC gages on the system and see what the pressures are at idle speed and compare them to normal specifications by the manufacturer.

Without having access to the vehicle to see what the pressures are doing, a definate diagnosis is most impossible.

Keep us updated.