Boy, we're really getting to the " bottom of the pit " here.
On most vehicles, not specifically M-B's, there is a signal ( either tachometer or oilpressure ) that keeps the relay in.
Sorry I can't be more specific or more help.
Just looked at my Haynes Manual & see the trigger wire comes from the ECU. ( Not too helpful ).
If you want to try something
temporarily, you could jump power from terminal 30 to 87 & see if the engine stays running.
http://img.eautopartscatalog.com/live/P202067087MEY.JPG