I am not familiar with this model MB, but it sounds like the seatbelt light in the instrument panel has been put into the wrong hole in the cluster (the fuel low position)
IIRC, the low fuel li a contact in the tank sender which can only close the circuit if the float gets to a low fuel level.
Please educate me if I am in error.

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Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.