Starter troubleshooting question
 

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?

At the starter..... You will need a voltmeter.

When you turn the key to start, the solenoid wire must have battery voltage.
Also the main battery wire at starter must also have battery voltage.
If that checks out, then either the main ground strap at the trans has a bad connection OR the starter is bad.

I have 12 volts at the starter and cleaned the braided cable (from transmission to body) and still get the same thing. Is there any way to test the starter to verify its shot?

Jump the wires directly from the solenoid to the battery power wire on the starter.

Don, recommend checking that the electrical connections at the solenoid/starter are clean and making solid contact ... disconnect and clean off any deposits. Also, check that the battery terminals and cable connections are clean.