Sounds like your dealer is not that good with a 12V system. There are only 2 wires that feed everything in that circuit (fuse). A B+ and a ground wire. Since most everthing in your BH walls is made of "WOOD", start with this. Remove everything from the walls and cabinets that are on that circuit. By remove, I mean remove the screws and pull them down/out with the wires still connected. I do believe that the white wire is B+ and the black is ground (I hope). A 12V test lite will tell you. Tug on the wires GENTLY and see if they have free movement. If one does not....the wires could be pinched together, screw ran through them(both of them), pinched together at a lite switch, or a lite. Do you know anyone that has a automotive type circuit tester. They work great for this. There is a quick way to find it.....put a paper clip where the fuse goes and look for smoke. By golly, you have found it. LOL BTW, it does take time to find these things.