The defroster grid on the rear window of your car is only painted on the glass. It gets damaged easily, and the smallest break in one of the lines can cause the defroster to stop working. Electrical disruptions also effect the performance of your defroster. If you notice patches of frost on your window after using this feature, you need to fix the problem but don't necessarily have to replace the entire rear window.
Check the fuse to make sure it's powerful enough to draw sufficient current from the car battery to run the defroster. Use a voltmeter to determine the strength of the fuse. Install a new fuse if both sides don't register voltage.
Ensure that the defroster grid wiring terminals are attached. One terminal is located on either side of the grid on the rear window. They frequently get dislodged and break the connection to the battery power. Fix them back in place with glue if they're detached.
Look in your car's manual for the electrical wiring diagram. Take out the defroster switch and see which wire leads into it and which one leads out to the grid. The switch is bad if the defroster works while both wires are jumped. Replace the switch with a new one.
Inspect your rear window for breaks in the defroster lines. Buy a defroster repair kit at any major auto parts store if you find breaks. Clean the defective part of the window with rubbing alcohol. Use masking tape to isolate the break, and apply the conductive paint included in your kit. Let the paint dry for 24 hours.