Open hood look in main fuse box for blown fuses or bad relays to heater fans.
Remove door panel 01 deville.
Use a flat screwdriver to pop the panel off.
The a c fan should come on only when you turn a c on at the dash panel only.
It has nothing to do with the heat.
Open the door all the way and roll the window down completely.
Theres one in the door handle lever cover theres two in the door handle pulley lift up to exspose them that will release the top half for the bottom half once you ve removed the top you ll see two screws exposed for the bottom half then remove the red lightcover at the end of the doors armrest its one there then remove the lightcover which may be.
Otherwise the problem is in the heater control module where you turn on a c and the heat that panel has a.
Here is how to remove the front door panel on your deville.
Remove manual lock knob.
Position window fully downward and ensure manual lock knob is in unlock position.
Feel free to contact me with any questions.
If the heater blower fans don t come on then look in the main box fuse for a blown fuse also.
Insert a flat bladed tool into the top service notch in order to disengage the locking clip 2 located at the front of the manual lock knob 1.
Look for all external screws you see.
Disconnect locking clip at front of manual lock knob by insertingsuitable flat bladed tool into top service notch.
Position the window fully downward.
Ensure the manual lock knob 1 is in the unlock position.