Not sure about a 1999 but on a 2000 the blower relay is on circuit C-108 in the Junction Box.
Check the fuses first and the blower relay.
Dealer.
The Heater/Blower Relay is on the blower motor housing under the dash in the passenger compartment on 1999-2002 models
My owners manuel list the blower relay as being in slot 12 in the fuse box under the hood on the drivers side.
The blower motor relay for high speed would be located in the power distribution box under the hood.
In a 1999 Cadillac Eldorado, the relay for the blower motor is typically located in the under-hood fuse and relay center. This is usually found near the battery, on the driver's side of the engine compartment. You may also check the vehicle's owner manual for the exact location and diagram of the relay layout.
Where is the blower motor relay at.
it is on the firewall and difficult but not impossible to get to and replace.
if it is the same number on it!!
Passenger side firewall near oil fill for the motor
Under hood fuse panel box. It is labled "Cooling Fan". To easily check it,swap it out with the "heater blower fan" relay and see if the heater blower works. Both relay's are made the same.
there is a resistor that controls the speed and should be near the blower motor its called blower motor resistor