Look into the main computer, that's where alot of those problems originate!
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.
Defective Blower Motor Resistor Pack.
the blower motor is located on the passenger side of the interior under the glovebox.
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.
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.
might need to replace blower motor
To change the blower motor on a 1990 Cadillac DeVille, start by disconnecting the negative battery cable for safety. Remove the passenger side lower dash panel and locate the blower motor, which is typically found near the firewall. Disconnect the electrical connector and unscrew the motor from its housing, then replace it with the new unit. Reassemble the dash panel, reconnect the battery, and test the new blower motor to ensure it's functioning properly.