my rear defrost isn't working and all the fuses are ok what could it be?

Here's how I approach this kind of problem.. I test each and every fuse I can find on the vehicle.. That's all fuse boxes and all fuses.. A lot of times there is a failed fuse that doesn't really have anything to do with a failed accessory circuit according to the listing!... I TEST all of them with a tester I have but a 12 volt test light will work. You'd be surprised at the problems I have fixed this way. IF all fuses are ok..use test light to check for voltage at rear window gird wire connector with key and defrost switch on.. OR have repair shop take care of it!