A 1998 Cadillac Deville does not have a blower relay. The system has a fuse and a controller to feed power to the blower motor.
bad blower motor
The blower motor relay on a 1980 Cadillac Coupe Deville is situated on the passenger side. It is located in the vicinity of the firewall and is just below the windshield.
Probably the blower motor, wiring to it or control switch
On the fire wall. Behind the engine.
Assuming that the a/c was in good shape before it suddenly stopped working, the problem is probably the blower motor. As it turns out, this is very common. Try searching answered questions under "blower motor 1997 Cadillac Deville" I just had to change mine in a 1996 Deville. Motor aprox. $240.00/Labor aprox. $100.
the blower motor is located on the passenger side of the interior under the glovebox.
Defective Blower Motor Resistor Pack.
With that year deville...its more than likely the blower motor
The blower motor fuse protects the blower motor electrical circuit. The blower motor relay operates only the high speed function of the blower motor. The blower motor speed resistor operates the lower speed functions of the blower motor.
might need to replace blower motor
A bad resistor or a bad switch.