Well I need the exact same answer. I have just finished rebuilding and painting the instrument cluster. All lights were replaced but my reserve light does not test. I have followed all the service manual tests. I do know that my reserve light contacts in the sending unit are bad. I cannot get them to work. So I guess its time for a new sending unit, crap $231 for the 240D.
But the light is supposed to test even if the sending unit is bad. The light, in fact all the lights that test when you turn on the key get there power from The cable in the back of hte instrument cluster.
Now where in the cable the lines go that I will have to find out. I tested the resistor and diode, they all checked ok. So there must be something else that is tied to the reserve light test.

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