The old 240 engine had loop plugs so I am guessing that the car is using the original relay. I may call up the dealer and have them run the part #. When I have access to the car again next week I will have a fire extinguisher handy and try plugging it in.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

Is it bolted to the emergency brake bracket? It sure looks like the warning buzzer relay to me (seat belt and key warning):

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41W21535NT&year=1979&make=MB&model=240-D-002&category=All&part=Seat+Belt+Warning+Relay

Warngerät means warning device.
Five bucks says it's the that annoying buzzer that tells you that you left your lights on, after you pull your keys out of the ignition, and open the door. (It could also do double duty for the seatbelts).

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

That's the one, it also controls the front dome light time delay on the 83 and later W123s.