TOO much cost...
That is a lot of money just to get a rear window defrosted and I realize where you live and the conditions. Here in the states there are electric heaters available to put in your car and with a little bit of ingenuity, wiring, a switch and ducting, a rear window defroster could be created. And for a LOT less than the cost of replacing the window. Most glass shops get close to $125 to replace the window in labor plus the cost of the glass. You could do the electric heater for less than the labor alone. A good weekend DIY job!!
Good luck...just my 2 cents!