a "difference in potential" or voltage difference can exist in a circuit where something is more positive then something else as well as the other way around. It doesn't have to be all positive and negative. It can be positive and less positive as well as negative and more negative. All there has to be is a "difference" and current will flow. If you have a high resistance anywhere in a circuit something downstream of it will be less positive then something upstream. Where ever it is, it's causing current to flow in the fuse. That Sea Talk circuit is seeing current at the bus or where it's tied into the circuit causing a path to ground which is better then the solenoid ground. I don't think of it as going "to" ground. I think of it as going "towards" ground and in this case I'm thinking it's going "towards ground" through the fuse. That's why, IMHO, I'll put my money on a high a resistance issue somewhere in the circuit. It could also be that the solenoid is pulling so much current that it's ground is not adequate and some of the current is pulling through the Sea talk's fuse. Remember, a fuse isn't directional. Like i said, just MHO.