Heated rear screen!

[Niall]

That's how its supposed to be....power through the switch, via the screen and then earthed via the chassis back to the battery.... You're putting the multimeter between power and earth so it will be live until switched off which breaks the circuit.

This.

You need to take the + lead off the rear window and see if you get 12v there. If not, move back to the next plug (iirc theres one behind the drivers C pillar). Get voltage there? If so, you know there is a broken wire between these 2 points. Take this principal all the way back to the switch until you find your voltage.
If there is voltage at the plug onto the window, you then need to check the other side for a good clean earth.