Heated windscreen only works for 30 seconds?

I have a 2002 Ford Focus, and the heated windscreen only comes on for about 30 seconds after I turn the engine on. The light comes on, and after 30 seconds it goes off and won't come back on until I turn the engine off and on again. I'm really struggling to find any answer as to why this might be. Any help will be greatly appreciated.

There's a thermostat turning it off because it does not need to be heated at that time.

It draws a lot of current, so turns off automatically

Hi so after 13 years of use the heated rear window might be broken so a replacement might be needed that means a new heated windscreen.