Is it getting a voltage at the demister connections? If it is and it isnt working check to see that the demister is earthing. I'm not familiar with your model but I think that its a hatchback??? If so your rear boot lid might not be making conection with the chassis to earth the window. Check it with a multi meter. If there isnt a voltage at the rear window then follow the wiring back until you hopefully find the problem. I think a demister heats the back window by causing resistance to the current producing heat. You should be able to read a higher voltage at one end and a lower one at the other. Please feel free to correct me here if my memory isnt the best.