Rover 25 rear windscreen heater problem HELP !!!!

Hi all , new here and looking for someone with a bit of knowledge !!!

My rover 25 rear windscreen heater does not clear my windscreen when it's misty :( .... The light on the switch comes on but does not clear ... I have changed the fuse and still no luck ..... Any ideas of what it could be so I know what to expect would be good ! ?

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