The tech rep that visited my shop quite often to help us figure out any problems with new or sometimes older Mercury outboards preached and I mean preached GROUNDS!!!!....I bet I find over half of electrical problems on most anything are grounds...Poor grounds can cause all kinds of headaches...Relays....circuit breakers can go berserk without the right ground...fusible links...overload contacts...switches of all sorts...I remember the Fords from the late 40's and early 50's that had a starter button that was completing the ground circuit thru the dashboard to activate the solenoid which was the grounded base type ..The solenoid was mounted on the fender well and the ground would be lost between there and the body.We would put a spot weld between the fender well and body and PRESTO!!...Even the button would lose it's ground to the dash...