[Liv2Fish]

Take the boat wiring out of the equation. Wire the base directly to the battery and do all of your testing there. If it works, you may have a poor ground or corroded splice somewhere. It's takes virtually no current to show voltage on a meter. LED lights draw almost no current too but at 24V, any resistance can trip you up.

Edit: sorry, I misread your reply that it's NOT LED. Make sure you have marine grade wire in place. My boat came from the factory with lamp cord between the bow light and switch. It lasted about a year. The ends looked fine but I was having issues so I ripped it out and found the insulation all cracked and wire was green inside the insulation. I Replaced it with marine grade 18/8 - 2 for what I needed and the rest for a remote control spot light. No issues since and its been almost 10 years.