From my research in trying to get mine installed I found a lot of messages on the MBZ.ORG diesel mailing list from people who had done that. You might want to check it (use Google's advanced search option to search diesel.mbzarchives.org). It seems you can just bypass the EGR computer completely and send the wires straight to the tach.

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)