it's FIXED!!!

within the boot going to the lid, there are 5 wires (blue, green (go to the lock), thin brown, thin brown and black and a thick brown). The blue and green wires were in perfect condition and the other 3 were all completely snapped. The wire in question that was causing the rear screen not to heat was the
thick brown one.
It was actually a right pain in the arse to do as all of the rear quarter trim, the boot lid trim and the roof lining had to be slightly pulled down as the wires feed in to the roof not the rear quarter panel as i expected.

I read somewhere you shouldnt 'fix' the wire in the boot but inside the car. So instead of simply re-joining the wires, i completely replaced and soldered a new wire in from the connector half way up the screen to inside of the roof lining. very fidley, but worth the extra time in the long run i think
But i do have a question for you guys though, the other two wires that were broken (thin brown and thin brown and black) o to a dead conttector (ie, its not plugged into anything). anywone know what that could be? i was thinking of some added option i dont have on my car?

hope this helps someone in the future wih the same problem!

Rich