Nissan micra heated rear windscreen, light comes on switch but not working?

Best guess?.check the wires that run from the body into the hatch,they are inside a rubber trunking but still can be damaged by the continual open /close of the hatch.If you have a multimeter there should be 12 v at the terminals on the inside gl that connect to the grid,If there is 12 v at the terminals then the grid has been damaged,A conductive paint is available that 'may' bridge any breaks in the element.