I have several light fixtures in my salon that are not working. They used to work when I first purchased the boat 2 years ago. They all work off of the same light switch (3-way). Now 3 of them work and 2 of them do not. I've tried replacing the halogen bulbs but still nothing works. I've taken the good fixtures and switched them over to the problem fixture and the good fixture will not work in that spot either. Appears as though I have no 12 VDC going to these fixtures but I have no idea how I would have lost this voltage supply and/or how to trace the wiring back to the DC panel. Any ideas?
You have to trace the wiring. You could have broken, corroded or burnt out connections. The problem could be either in the power or ground wires. If you don't want the trouble of tracing the wire hire a marine electrican.
When you say "appears as though", are you checking voltage with a meter or just by plugging in fixtures? If you are going to start DIY on electrical anything, get a multi-meter and learn how to use it, they are cheap and easy to use. Chances of corrosion or cracked connectors are much higher than something behind the walls coming apart, examine your connectors carefully. You may have wires going in two or more directions from the back of the light switch, another easy place to check. Usually if there are connections in wires behind walls, they are made at the fixtures and switches, not randomly placed at inaccessible places. If you can pull the fixtures out, you can usually access connections to other fixtures there.