Check that the wire on the rear defroster is connected to the defroster. If broken off repair it.
There is a repair kit available to repair the grid. A test light will find the break in the grid.
AUTOZONE.COM click REPAIR INFO, then click, REPAIR GUIDES
go to auto parts store and purchase a haynes repair manual
the best thing to do is take it to the dealer. Trust me.
You need check it. Very often it gets scratched and as a result damaged. If it's damaged even in one place it will not work. If it's the case you can repair it using a special defroster repair kit which you can get from Advance Auto parts. Check the fuse too.
You call an Auto Glass repair shop. This is not a DIY job. Your insurance will almost always pay for this repair.
Find an armor vendor and press the repair button in the transaction window.
you can hold the menu button along with middle button hold it for 10 seconds
Check the fuse first. Then check, with a volt meter, to see if there is 12 volts D.C. to the defroster at the place where it is plugged in. If you have voltage then look at the defroster heat strips and check for a break somewhere. There are repair kits for a broken power strip. If the fuse is good and you have no voltage, check the wiring, switch, & relay.