If you have a light on the lid of your trunk you already have a ground. Take that screw out, clean it up and reinstall it. See if that helps. Then use test wires (two alligator clips connected to lengths of wire) and connect that screw to the body of your amp. See if that makes the buzz even less.
You can install a permanent ground in hidden areas. This guy here is installing proper grounds on existing connectors inside his trunk. Installing the new ground wires - YouTube