Sorry if I'm stating the obvious here but there are a few things I'm not sure you've done yet. Have you attempted to find where the wire terminates? It has to go somewhere. Have you counted all your accessories and accounted for them on the switch board? Bilge pump, cig lighter, fish finder, trolling motor, courtesy lights, radio, ship to shore radio? Just throwing some out there. Do they all work? If each component has a switch and the switch is activating them then I would move on to checking for old components that have been removed. Maybe the wire is hanging loose in the back of the boat where a trolling motor used to be? Maybe there are a set of lights already in the boat that aren't working? Maybe the switch is bad, have you checked the switch? Maybe the switch is already wired through the ignition? Have you turn the ignition to on and then tried the switch? Bottom line is that if the switch has a wire coming from it, the wire has to end somewhere. Without physically being in your boat noone here can tell you why it is wired already or where it goes. It unfortunately will take some time on your back or crouched over following the wire. You might have to cut a few wire ties and maybe loosen some clamps but we need to know where it terminates before you can get any real advice. I wish you luck. My first car was a parts car for the guy I bought it from. Instead of getting up into the dash and pulling the plug, he just cut the dash out of the car. I had a bunch of fun rewiring the dash plug into the car. I'll tell you that it was super rewarding having it all work (I was 14). I have found some weird wiring in some cars in my day. I would check the switch for things that might seem redundant like maybe the switch turns the float on the bilge on and off. Hope I gave some ideas to chase down. Good luck