If you used an aftermarket switch, (i.e., Febi, etc.) trash it and get an original part form the dealer. Same thing happened to me years ago on my 124 and replacing with an original part solved the problem.

For some reason, ohmic switches (which I believe the NSS is ) are not made well by aftermarket suppliers.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black