The first thing is to unplug the demister switch, it is a 3 pin plug. Test that you have 12v power at the blue/red wire. Then test the white/black wire is connected to earth. Then plug the switch back in and see if the light in the switch comes on. If it does not, you have to carefully take the switch apart and clean all the metal contacts with sandpaper.
Done this myself only days ago.
Once that works, you have to start looking at the actualy demister on the window...