My 02 sedan sat for a couple of weeks and then wouldn’t start. No issues before - started every time. After electrical troubleshooting it turned out that the starter / solenoid was bad. Actually the connection between the starter / solenoid was a soldered connection and it developed a crack. So both were working, but the starter was not getting power. I replaced with a spare from my parts car. I doubt that is your problem, but testing each component is fairly straightforward to identify if you’re getting power to the parts.

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07 E350 4Matic wagon, 06 S430 4Matic, 99 SLK 230
retired W210 (x3), W124, W126