A 2009 Mercedes C300’s battery location is under the hood of your car, on the passenger side of the vehicle. It’s underneath your cabin’s air housing, underneath a plastic panel. To access the battery, you will have to unclip the plastic panel.
The battery will be closer to the top of the engine, where the front windshield meets the hood of the car. Mercedes batteries can be notoriously tricky to remove and replace yourself, so if you’re struggling with it and don’t want to risk damaging your car, taking your car to a Mercedes professional is the next best option.