rear window defroster

My wife's '82 300sd defroster doesn't work, but the light does go on when I flip the switch on. I'm hoping it is a simple fuse problem. Can anyone advise?
Thanks
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Replace fuse. It has two different circuit that there is also a relay to route high current to rear window. So light on switch doesn't mean power to rear windows.
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I have the same problem on a 1988 420 sel. I replaced all small fuses in the fusebox under bonnet. Is the high current fuse the long black one ( abot 5") in the same box that might need replacing??

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