Unless it's wired to a source that is always hot. You could disconnect the battery and see if the needle stays in the middle or drops toward D, right?
I think that I would disconnect the power wires related to the fan set up and then see how the ammeter reacts. I think that would tell you if it was the fan wiring that was causing the issues.
I can't understand that there is a hot wire from the battery terminal AND a hot wire from the starter relay all headed for the fan...
