Well, Just serviced a boat with multiple issues. Trim not working, Helm lights on but dim, courtesy lights remained on no matter what, vhf not working. After much searching found ground bus at the helm had daisey chained poorly connections were compramisd. What was happening is the devices were looking for a ground, and it was getting a back feed from some other source. Repaired all the grounds and everything went back to normal. You will see strange things happen with a bad ground. As in trailer lights, when something isn't working you blame the bulb/fixture, when it's simply the grounds that are causing the strange behavior.