I think I'll take a shot @ building my own.

While not specifically about W124's there's a lot of good info here about Scanners including a good diagram to make your own:

http://pages.prodigy.net/jforgione/MB_X11.html

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Currently:
1972 350SL Euro 4spd
1973 BMW R75/5
1981 BMW R80GS
1995 FZJ80 with OM606 Conversion In Progress