If you have a multimeter, You can check between the positive connector on the back window (there may be one or two connectors, one either side of the inside of the window, at the top) and then use the body (not any painted part) as a negative and see if you get a reading when it's on (someone could sit inside the car and press the heated rear window button and if you have the multimeter on the "buzz" setting it should buzz when they are in the "on" position, if they don't then the connector isn't getting power obviously). I'm unsure if they are fused or not? I've never had them go on me so not looked into it really.