Voltage is supplied to the heated rear window switch only when the ignition is on. When the heated rear window is switched on, the orange signal lamp lights up in the switch button.
If the heating element is damaged, replace the glass together with the heating element.
Helpful advice. On sale there are special kits for restoring the filaments of the rear window heating element. Instead of replacing the glass, you can try to restore the heating element filaments, following the instructions that came with the kit.