The cabin lights on my H-30 are wired in parallel, The positive wire from the breaker panel and the negative from the negative block next to the breaker panel go to one fixture and then the next and the next and so on. There are two criimp fittings (one for the negative and one for the positive wires) under each light fixture. Each crimp fitting has a wire coming from the last fixture a wire to the current fixture and a wire to the next fixture.
I would bet that the connections in the fixture are bad or the wires under the crimp fittings are corroded. I had this problem on my boat. I took the fixture down, cut off the crimp fittings, stripped the ends of the wires and put on new crimp fittings. While I was at it I replaced all the light fixtures with new LED fixtures.
If you remove the old fixture (on or two screws into the headliner) you can jiggle the crimp fittings and if the light goes on an off you probably have corroded wires in the crimp fittings. If the connection to the first light is bad the lights further down the line wont work either......
