I had this exact same problem. It turned out the heating element was not rounded.The grounding point for the heated rear windscreen is on the right right hand side of the boot frame if you are looking at it from the rear of the car.
You will need to remove the plastic trim and I found that there was a loose wire hanging there not connected at all. I crimped a loop connector onto the cable and used a self tapping screw to connect it to the frame. Works all the time now.
Martin
December 2009