wire. Everything has to come back to the negative. You won't miss anything that way. Have you measured the resistance between the two ends of the bonding wire connecting the rudder to the bonding system? It should be almost zero. You could have an open under the plastic insulation. Of course, you have cleaned and inspected the connection points. Is it a spade rudder or does it have a gudgen attaching it at the bottom? You might have some kind of an electrical loop going. That could indicate that the webbing inside the rudder has broken down and you have a "difference of potential" between top and the bottom. Jumper the rudder top to bottom and check the meter and see if it changed. If you have open between the system and the rudder you might be getting some current from another boat or the dock. See if your boat neighbors are having any issues. These are just off the top of my head. You might want to invest in a tester like a "Salty Dogg". I have one and it's pretty good. Good luck.