2003 Chevy Monte Carlo No defrost

Check your blower motor/switch/wires/ground/connectors/fuse/relay etc.

Check for voltage at motor. If you have enough voltage at motor but motor doesn't come on, you either have a bad motor or a bad ground connection.
If you read a low voltage with your voltmeter, you have high resistance in the circuit. You'll need to do a voltage drop test throughout the circuit to find the problem.
If you don't have voltage at all, you have an open somewhere in the circuit. Look for voltage working your way back in the circuit until you find voltage. Once you do, replace bad component.

You'll probably need a wiring diagram.