What he ^^ said, first check you have power at the wires each side (Incidentally there are two heating elements in the screen) if you do then the screen itself is faulty, which unfortunatly are quite expensive!

If not the relay may be faulty, clearly the fault is after the switch due to the fact the light illuminates so you have power there!

They do sap alot of power and so you should be able to hear and feel it power up by a change in the engine note when idle, due to the electrical demand that has been applied!

Martin