Short circuit 63 Dodge Dart
In the shop manual is a wiring diagram.
The spark indicates electrons are moving, since it sparked and melted the fuse you tried between the bulkhead connections and the feed wire, the short is downstream from that. This assumes everything was turned off.
The battery power is always available in the wires connected to the battery positive.
So a short to ground can occur if any of them get pinched or have the insulation chafed off.
Also inside the alternator or touching an ammeter post.
The ammeter shows electrons moving in or out of the battery. Think of it as current just like a stream or river. It will show discharge when a circuit is completed and current is flowing out of the battery.