1. Fuse: Check the fuse for the rear window demister. It is usually located in the fuse box, and it may be labeled "Rear Defogger" or "Rear Window Defogger." If the fuse is blown, replace it with a new fuse of the same amperage.
2. Switch: The rear window demister switch may be faulty. Try turning the switch on and off several times to see if it makes any difference. If the switch does not seem to be working, it may need to be replaced.
3. Wiring: There may be a problem with the wiring that connects the rear window demister to the power source. Check the wiring for any damage, such as cuts, breaks, or corrosion. If there is any damage, it will need to be repaired or replaced.
4. Demister Element: The rear window demister element itself may be faulty. This element is a thin, metallic coating on the inside of the rear window that heats up when the demister is turned on. If the element is damaged, it will not be able to heat up and clear the rear window. In this case, the entire rear window will need to be replaced.
If you have checked all of these things and the rear window demister is still not working, it is best to take your car to a qualified mechanic for further diagnosis and repair.
