Welcome to Iboats. I don't have any direct knowledge of your particular system so I may not be much specific help, but I hate to see a request for help go unanswered. After a half century working with electrical circuitry I've learned that the first rule to resolving any electrical problem is getting reliable test data before making any assumptions about what the problem might be. Yes, your problem could be in the ground (negative!) side of the circuit. The negative side is every bit as important as the positive. Unlike in a car where you can usually find a reliable ground/negative path through the frame to use as a test reference, a boat has areas of limited access to what may or may not provide a reliable ground test point.
One way to avoid confusing test results is to use a super long "extension" to your negative test lead connected directly to the negative battery post. If that lead is long enough, you can test every positive point in your system, knowing that you're getting reliable info. If all positive points test live 12V where they should, then suspect the negative. Start the negative test by switching on a simple light duty circuit such as navigation lights. You can then use your super long test lead as a "jumper" to temporarily provide a reliable ground by connecting it to successive negative terminals throughout the boat until the nav lights come on. The problem is often loose or corroded terminals that should be cleaned, tightened and protected from further corrosion with dielectric grease. - Grandad
