Under the hood, passenger side firewall, close to the heater blower motor.
You have a bad blower motor resister.........
right next to the blower motor I'm don't know about the 2000 model, but it is directly behind the blower motor of the 1999 Century.
how to change the blower motor
The blower motor is under the glove compartment
the blower motor relay is located next to the blower motor it is connected to the motor by a purple and black wires
Loose connection or blower motor is bad
Your blower motor resister, which is a circuit card located in the bottom side of the blower housing ( just over your front passenger's toes ) is most likely partially cooked - replace it.
Its located under the hood on the passengers side mounted in the a/c housing assembly.........
In the dash under the glove box
Fuse is blown.
down uner the glove box
No repairing those blower motors, replace it with a new one.