Quite possibly was done by someone not knowing how to run a ground, trying to eliminate interference in the depth finder, in which case it should have been connected to the ground side of the cranking battery. Do you have one of the older 24/36V Motorguides on the boat? Are you running a 36V trolling motor system and if so, are you using the cranking battery for the “third” battery. If so, you cannot run the common ground mentioned above with causing a short. It is definitely something another owner did and I would suggest removing it on both ends.
John
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