There's a (largish) file that is a book of trouble codes for MBs at:
http://www.geocities.com/MotorCity/Downs/1039/mercedes/cs1000_mb.pdf
It's about 87 pages, so I printed it out. It has the connector pin layouts, plus codes for each subsystem for the various years.

For cars earlier than '95 that don't have the standard OBDII interface, check out Jim Forgione's website for lots of info, but especially the bottom of the page at:
http://pages.prodigy.net/jforgione/MB_X11.html for a schematic of a simple code reader you can build for $15 worth of Radio Shack parts. It was a lifesaver for troubleshooting my 92 400E's air conditioning problems.

Gerry Visel