Check the connectors on the base of the tailgate and the plate it connects to. This is where the rear screen demist is powered from. they are on springs and they sometimes brake after a lot of use or the plug under the plate burns out if it has been wet. Can't remember off hand if it is the 2 or the 4 connector that powers the screen demist but it is easy enough to check both.
Alternatively the wires that fasten to the glass may have become damaged or the lines on the glass may have all been scratched so the current can't flow through.
Rachel
July 2007