* Blown fuse: This is the most common cause. The light might be illuminated, indicating power to the switch, but a blown fuse prevents power from reaching the heating elements in the rear window. Check your car's owner's manual to locate the fuse box and find the fuse for the rear defrost. Replace it with a fuse of the same amperage.
* Faulty relay: The relay acts as a switch for the high current needed by the defrost. A malfunctioning relay can prevent the defrost from engaging even if the fuse is good. This requires more automotive knowledge to diagnose and replace.
* Defective heating element(s) in the rear window: The thin wires embedded in the rear window glass can break or become disconnected. This is often indicated by a section of the window not defrosting or a lack of defrosting across the entire window. Repairing this usually involves replacing the entire rear window.
* Wiring problem: A break or short in the wiring between the switch, fuse, relay, and heating element can interrupt the circuit. This is the hardest problem to diagnose without specialized tools.
* Faulty switch: Though less common than a fuse, the switch itself could be malfunctioning. It might show the light as on while not actually sending the necessary power.
Troubleshooting Steps:
1. Check the fuse first: This is the easiest and cheapest thing to check.
2. Check the rear window for visible damage: Look for any broken or loose wires in the window.
3. Listen for a clicking sound near the fuse box or under the dashboard: This might indicate a relay problem.
4. Consult your owner's manual: It will have a wiring diagram and fuse information specific to your car model.
If you're not comfortable working on your car's electrical system, it's best to take it to a qualified mechanic for diagnosis and repair. Attempting repairs yourself without the necessary knowledge can be dangerous.
