Its not working. Why? Its not the fuse. The button lights up when it pressed. The wires are connected to the screen.

"I'm Out!" of ideas

The first thing to check would be, are you getting 12v at the wires connected to the screen. If you are then its the window element thats broken, and tracing that is almost impossible.

When you press the heated rear window switch, do you hear the relay click?

As Mannyo says, check for 12v on the connectors on the boot glass. If 12v is present then the element is probably faulty. However if there isn't 12v then either the switch, fuse, relay or there is a break in the loom somewhere.

How did you "prove" the circuit? As others, you need to actually show a supply at both of the connectors to prove the circuit.

Oh... you are aware the rear window heater only works when the engine is running, to avoid battery drain.