Can I upgrade my rear view mirror with one that has homelink and garage door buttons. Is the wiring harness there

Hello, you'll want to check directly with your Mercedes-Benz dealership. They will have an "accessory" listing that will have items such as this (if applicable) to be used for your C300. I can't say specifically for Mercedez-Benz, but typically the Homelink / Garage Door Buttons don't require additional or separate wiring.