I would check the battery and grounds first. Check that the battery has correct voltage and cold cranking amps. If all that checks out, I would look for water entry and corrosion in the drivers footwell where many positive wires come together in a block. Also check the blocks near the battery, you may have to remove battery to access. My car is a w124 but w126 should be similar. Also could be a diode in alternator went bad.

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1990 300d 2.5 turbo
1995 E300d