If the light comes on and the relay clicks the relay is fine! The light takes its feed from the same pin that outputs to the elements.

The heated screens are prone to stop working. I suppose you might have a broken wire.........

Have you had a meter on each of the wires at the side? Each one should have 12v (2 seperate elements)

AND.......remove the roof clock and you will find the earth strap, and also a rather annoying burn mark on the back of your clock..........