Rear Window Defogger

The rear window defogger on my '79 300D has stopped working. Before I start replacing components, what are the most common causes of these failing, and where best to start?
Thanks in advance...
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The 99.9% failure is the strip fuse in the fuse holder by the top of the drivers front shock.....

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Thank you very much. This sounds like a quick fix for a change, and I appreciate the help.
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wrote:

Is that a glass fuse inside the square box on top of the wheel well, or is it a fuse inside the fusebox on the firewall aft of the wheelwell?
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It should be a 30 amp strip fuse in the holder. Here is what a strip fuse looks like: http://www.flickr.com/photos/aufevervw/1510376195 /

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Hmm. Well, I didn't see anything in the fusebox that looked like that, but then again, I didn't know what I was looking for. I only saw the typical colored white and red fuses. I will analyze this again in the morning. Thank you--this is most helpful, as I would never have know what to look for otherwise.
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Read what I wrote:
The 99.9% failure is the strip fuse in the fuse holder by the top of the drivers front shock.....

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I checked again, and the only thing near the top of the wheel well, driver's side, is a square plastic box, with two large relays inside, and a plastic in-line fuseholder with a very small red and while glass fuse inside. No strip fuse(s). Also, there are no strip fuses in the fuse box on the driver's-side firewall. I am at a loss as to where this 80A strip fuse is located on my car.
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79 123 body should be fuse #10 in the main fuse box. :)

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