That was the problem with this boat, much of the wiring (minus the main wiring harness) was replaced by previous owners. We're pretty sure it is the ground for the fuel filler tube. The only way to confirm this is the remove 1 or 2 panels, and by that point he was out of time and I was out of money. He worked on that electrical system for 2 days, charging by the hour, and that was all my budget could handle at the time.