Didn't 97s have the biodegradable wiring harnesses? One of my cars had the same problem with batteries going dead in a few weeks. Also failing o2 sensors. Took me years to discover the bad spot in the wiring harness. There were no outward signs. Just a slow power drain.

Good luck

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1979 240D, 4spd manual, Power Sunroof, manual windows, 147k miles, Pastel gray/Black MB Tex.
1991 300D 2.5 - Smokes like it's on Crack!