From the clues you provide -- which are really very little to go on -- it would seem that someone attempted to use the metal tank as a source for the negative return for the autopilot.
That's a no-no. Autopilot motors (and, in fact, virtually all connected devices aboard) need their own positive and negative feed wires. It's generally not OK just to "connect to a ground somewhere."
Also, the "wires unattached" in the plural is worrisome. Normally, there'd only be a single ground wire.
With more info on your setup, it might be possible to diagnose the problem if the above isn't it.