How do I troubleshoot my rear window defroster? on 2007 Audi A8 Quattro

When I press the dashboard button for the rear defroster, the light comes on and, the outside mirror defrosters, which are also controlled by the same switch, begin to defrost. I have no indication the rear window defroster is working no matter how long I wait. I don't see a fuse specified in the owners manual. Can anyone help me with this? The window grid appears to be intact.

Asked by for the 2007 Audi A8 Quattro
maybe there is no power to your defroster
this reply states the obvious. the question is how do i troubleshoot it?