My interior cabin lights operate on a manual slide switch. When I remove the bulb and check the voltage with the switch on I get 12 volts. When I turn the switch off and replace the bulb then slide the switch on the light will not come on. You would think the bulb is bad. But when I crank the boat and slide the switch on the lights come on. All the lights in the cuddy operate this way even though it is show 12volts with the engine off. What is making it work like this?