We had the same problem.
It took long time to solve , because the light was not always ON.
The way to find where a bad contact between the minus and the grounding system is, is to start from the main switches near the batteries, disconnect each of the cables while checking the light. Ones you find a cable that if disconnected the is OFF, look for the equipment down the cable and check the contacts and put corrosion x.
We found the problem in the forward locker . We had a leak of water from the new Bad! installed windlass rubber deck switches that created humidity on all electrical motors in the locker.
Good luck with the challenge.
You can call me if you want more info.
A 54 160 2010