I think your ground connection
You said your ground connection was origionally to the fuel tank, which is of course the first no no. After you had the new tank installed, and dangling wires, no auto pilot. Then a hook up and a display, but no drive motor. I think you have a poor ground connection, that allows enough current for the display, which takes very little power, but not enough for the motor, which takes 3 or 4 amps. I am in total agreement that you should go over all the wiring. Anyone who would use a fuel tank as a ground, would do just about anything.
Dream like you will live forever. Live like you will die tomorrow. ( James Dean )