I've seen this battery cable problem many times in the W210, there is a through connection in the driver side firewall and the cable from this connection to the starter corrodes. What i do is replace the through connection and the cable to the starter.

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600SEL '91
300E 4Matic '88
240D '83
280SE '77
350SE '73
The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

i don't believe in the lord! He's never bought me a Mercedes Benz.