Try disconnecting the wire at the generator Arm. terminal, and see if your short goes away. If so, the cutout relay points in the regulator are probably stuck together. That's a not-uncommon problem with generator/regulator charging systems, and it usually damages the regulator beyond repair. Once the points stick, there's enough damage to them that even filing or sanding the contacts doesn't guarantee it won't happen again. The good news is, if it's still charging, the generator hasn't been damaged.
Most replacement regulators are not polarity-sensitive, so positive or negative ground shouldn't be a concern. If the short is not through the regulator, just start disconnecting circuits one at a time until the short goes away. It's possible something has been connected incorrectly and it's drawing current from the battery continuously. The generator/regulator circuit is the most likely suspect.
Jerry
