I agree with makomark, lights, radio, bilge pump should all be powered with a switch that way you can sit in the middle of a lake at night with your at anchor lights on and the engine off. You mentioned old wiring, that also might also be part of your problem. Your problem may be corrosion, poor ground, faulty switch. Are you getting 12volts to the fuse panel? Are you getting 12volts to the switch? Any connections in the circuit? If so pull them apart and check for corrosion. Any crusty brittle wires or sections or sections of wiring that are brittle? Any inline fuses? Pull them apart and check for corrosion. Battery posts nice and shiny? I hate checking for electrical gremlins, you need to be a contortionist. Take your time and approach the job in a methodical way.