Your, I'd start by hooking up a voltmeter to the defroster wires in the back, you know, the ones that attach to the heater element. Then, activate the switch and see if you have juice at the element. If you do, then the element is bad (unlikely), if you don't, you have to find out if there is a separate relay for the window apart from the one for the mirrors. I do not know the wiring for this car, so someone else better help you here.

Good luck