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.

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1 answer 1 comment
maybe there is no power to your defroster
this reply states the obvious. the question is how do i troubleshoot it?