If your rear view mirror heating works but the rear window is not heated then you should try the following.
Locate the yellow connector on the left of the fuse box (don't know where it is on RHD cars) and open it. If the central pin is blown then that's probably it. The pin used to power the rear window is way too small for the current it has to drive.
The best is to bypass the connector and to use a larger single wire connector. Or to solder the wire, you won't dismount your car every day.