IF it were I, I would strip every last bit of original wiring
from the boat and start fresh with a layout I totally understood from start to finish. The wiring
you have has probably evolved and been added too over years and mixed different sizes and systems. The original owners probably were happy and understood its eccentricities and knew what not to touch. It shouldn't be like that and wont be if you bite the bullet and totally rewire. Don't think of it as a potential dangerous pain, think of it as a great chance to get your boat sorted your way.