There is a wiring color code for boats and Boater's World sells colored wire by the foot.
Biggest thing is to be sure the wiring is sized right and fused properly.Using tinned wire helps alot and be sure to use marine grade connections.Expensive but well worth it in the long run.
Try to find the book- "12 volt bible" - or something along that line.It shows alot of good tricks and troubleshooting.
90% of 12 volt trouble will be a bad/dirty ground wire.
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