If you already didn't find the switch, it's located on the left side of the transmission, just in front of where the shifter linkage attaches. There's an electrical connector facing forwards on it.

I found great help in this DIY-guide to repairing the switch (although in my case the switch wasn't broken, but it feels good to know it's now fully serviced!).

http://www.peachparts.com/shopforum/showthread.php?t=112452

Regards

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Tomas, Sweden
1966 Mercedes Benz 230S with OM617.912, automatic. Disk brakes from W108
1983 Mercedes Benz 300TD grey, OM617.912 and 5-speed manual
1983 Mercedes Benz 300TD blue 7-seater, OM617.912 and 5-speed manual