There are 2 metal tabs on the back window that coiled wires plug onto. This lets the rear window move yet still have the electric defroster. Check these connections, that they aren't loose. Also check that these 2 tabs are still firmly in place on the glass. I knocked one off one time when cleaning the rear window (overzealously... lol). Next, check for voltage at the connectors. Then, check for continuity across the defrost element by placing a meter on both of those tabs (with the wires removed, but be very careful when taking them off) somewhere in what I've said you should find the problem.