My first recommendation is to get that puppy and all those areas cleaned up first (Wash, Rinse, Leaf Blower & then Shop Vac) so you can actually see everything first Brother
Some stripper soap (the type used to remove paint sealant / coatings) that is much stronger than most on the market will come in handy!
This also works wonders after you lay down some soap and get it all cleaned up via normal wash chemicals as a follow-on step. Just be careful with it and wear shoes and safety glasses! And just do not leave it on for a long period around all that wiring (5 to 8 min) and rinse it off real well after application to those respective areas...
http://www.starbrite.com/item/mildew-stain-remover
OK (two days later and the boat looks almost new) now you come in and see everything, it's real condition, and the respective wire color codes. In theory, they were wired to ABYC standers thus should employ the same basic wire color code specified for each system. You can find that guide here:
https://www.pacergroup.net/abyc-color-code-standards/
I would then go though and clean and reattach every signal wire at the switch panel, and all the grounds back at the engine, batteries, and main power / A-B switches she has... After all that is done, now go grab the meter and see what is still dead and needs replaced
Chasing after wiring issues in what looks like bilge of a oil tanker (after) a Coal Mine explosion, is not fun and really in many cases a waste of time since you cannot really see anything and trust me, you are going to find more wrong or it just crumbles in your hands while working on it...
Hope this helps!