In the 123 ( and I'm sure Mercedes would follow the same method of design) the sender has a float that when at the bottom, IE empty, makes contact with 2 probes on the bottom. This sends the signal to the guage light. However the reserve light test curcuit goes thru the seat belt buzzer module. So determine if the illumination is due to the seat belt module or the sender.
To test the seat belt module disconnect the buzzer module, just after turning the key on. This is when the test of the light is done. If it is still on then disconnect the sending unit. Once the car is started, which turn off the test curcuit
Where is the seat belt buzzer module in your car? I do not know.
Also the test curcuit is dimmer than the actual low fuel mode.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car