I have a 1999 C230 that sat for several months through the winter. A month ago, I charged the battery and it started fine. It sat for another month and now all I get is a click on the starter (and I re-charged the battery again, didn't help). I'm guessing I have a bad ground somewhere, but where do I start?

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2008 ML350
2007 Honda Element
2007 Honda Accord
2009 Honda Civic