I own a 2009 Mercedes C300 and it works fine most of the time. However, anytime there is moisture in the air or it rains etc, all of the lights inside outside on the dash start flickering like crazy. Pretty much anywhere there is a light. Has anyone encountered this issue? I believe its a ground issue, but I checked the ground wires I could see in the engine bay, and they seemed good. Any help would be much appreciated.

 

Have you checked the fuses as well? I'm attaching a link to a DIY tech article from our site, it may be a long shot; but the fact that it only happens when there is moisture may be your culprit.

Mercedes-Benz W204 Rain and Light Sensor Replacement - (2008-2014) C250, C300, C350 | Pelican Parts DIY Maintenance Article