The easiest way is to use a cheap volt ohmmeter and check the voltage
where the wires hook to the gas struts that hold the window open.
You should also check the (4) spade connectors for corrosion and make
sure they are seated tightly, also, check where the wires from the
struts connect to the heater element.
If they are loose or broken off you have to re-connect them. Some people
have had luck with electrically conductive glue, on mine I had to use a
low wattage soldering iron (40) watts and fine rosin core solder.
Also check the wire grid on the rear window for breaks.
Repair kits are available at any auto parts store.
You can get an inexpensive volt/ohm meter at Radio Shack for less than
Work backward from there, check the fuse and then the switch.
Good luck and I hope this helps.