I will thoroughly go over all connections, switches, and splices visually and with a meter while power is engaged. Depending on the amount of electrical applications of your craft this could take 1 to 5 days. Investigating in this manner should should pin point the nature of the problem and present potential solutions. Often, in water craft, it is an issue of corrosion and an arc-out in the line that leads to unpredictable electric activity. Sometimes the electricity will flow, sometimes it will act sporadic and "flash" through electrical signals (lights, horn, etc.) Being able to find the specific spot of corosion is the only way to resolve the issue.