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Wiring and old boats
Buy a good book on Marine Electrical Systems. Purchase a copy of ABYC's electrical codes.
The battery(s) POS will lead to the No.1 and 2 terminals on the battery Selector Switch or the DC isolation breaker, if you have only one battery. From there it should go directly to the POS bus bar on the distribution panel. Each circuit (system) should have it's own breaker and/or fuse to protect the POS supply to the "Load", i.e., what is being powered. The first thing you need to do is hand-over-hand each wire to: 1. know it's condition and 2. know what load it is supplying. The battery NED (-) cable goes to a NEG (-) bus bar or terminal block(s), where all load NEG leads terminate (or should). Terminal blocks may be "ganged" together, i.e., there is a "jumper" wire from one to another or even from one or several, terminals to another on the same block. Terminals that are jumpered together are the same polarity - look closely.
Start by drawing an elevation (looking down at the deck) diagram of your boat. Show the location of the panel and all electric loads. draw a diagram of the back of the DC panel, indicate every fuse and identify the load side of the fuse with the label on the front of the panel. Pick a circuit to start with, label the load side wire at the panel with a number tape (do not use "dime tags"; the round tags with a string, they have a metal band around the cardboard tag and conduct electricity), indicate which number you used on your panel diagram. Hand-over-hand that wire all the way to the load. If there are terminal boards in the circuit tape the lead on both sides of the terminal. Identify the terminal board as a POS terminal with a POS or + wire tape. Everywhere there is a wire division, tape the wire with the number and the polarity (+). When you finally get to where the load is, label the wire at the load. At the load, there will be another wire that will be the NEG or (-) lead. Label it with the same number as the POS lead but add a (-) or NEG tape. Hand over hand this wire the same way as the POS wire. There may be terminal blocks in the run also. It should terminate a NEG bus bar that the battery NEG lead should be attached to. Here is where it can get dicey as there are several ways to ground the battery and the NEG bus, some go to an engine, some to a ground plate, some have an isolated ground. On your elevation drawing of the boat, indicate the location of all the terminal blocks you encounter and the "run" of the wire. Do this with all your circuits using a different number schema for each circuit. Laminate and store/post the elevation and panel drawing as near the panel if possible.
Some problems you may encounter: More than one wire connected to a terminal block. Pick one and follow it to where it goes number it with original number and if you have them a letter tape, e.g., "1A" and indicate the new number on both the panel drawing and the elevation drawing. Dead ended or clipped wires. Disconnect these and remove the wire, be careful, some of these that appear to be clipped may be just broken from the terminal lug.
After you know what you've got, you can now decide what to do about bad switches, wire, dangling unloomed wires, etc.
Hope this helps.