I have a 96' S420 and tonight I turned the rear window defog on like always to clear the window. It only takes 4-5 minutes so I always manually turn the switch off myself. This time it would not turn off. I stopped the car and turned the car off, then cranked it back up and the defog was them off. But now it will not turn back on??? I checked the large 40 amp fuse in the trunk and it is fine. What has happened here???

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Porsche's for Breakfast & Ferrari's for Lunch

... but when a circuit stays hot when you try to shut it off, its usually a relay that's stuck.
Anyway, looking at the defog circuit, there are two relays involved. N10, which is the "combination relay" (includes turn signals, wiper motor too) and K13/1, which is the rear window defog relay.
The circuit is really not too bad and a little time with a meter will find your problem.
N10 is under the hood in the large fuse box near the master cylinder.
N13/1 is in the trunk on the right hand side.
When you say it wouldn't turn off, I assume you mean the yellow light on the pushbutton control unit wouldn't shut off. Unfortunately, K13/1 runs off of N10 in this circuit, and its not clear which relay generates the signal back to the light... so, that's not much help. You'll have to troubleshoot it unless you have known, good relays to swap and see.
I'm gonna guess its N10 simply cuz its the largest, most complex, and undoubtably, the most expensive. :-)
Hope this helps some, KenP

But if it was N10, the turn signals and wipers would not work too right? Where is the N13/1 on a 93' 500SEL as I can remove it and try the swap? Thanks for the good information!

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Porsche's for Breakfast & Ferrari's for Lunch

I'm not positive, but based on the size of N10 (from the pictures), I'm guessing it has a number of relays/electronics in it... not a single relay. So, failure of one would not necessarily affect other circuits. Some of the techs here may know.
I'm afraid I don't have access to the info you seek on your '93 SEL. I've only researched and gathered info on the '96 S Class cuz that's what I have... sorry.
Anybody know?
KenP