Ralph,
Too bad about your problem. The battery feeds the starter and the large wire that connects to the ammeter. When the battery is connected, there is constant, un-fused power running from the battery, through the ammeter all the way to the voltage regulator and to the powered side of the key switch.
Because your damage was limited to the wire from starter to + terminal on the ammeter, and not through the ammeter to any wire on the - side, it tells you, as you observed, that the short was only in the wire from starter switch to ammeter +.
You said the ammeter case itself is burned and black. To me, this would indicate the source of the short to ground. The wire melted because it was passing more current than it was designed for, and the source of ground for the rush of current sounds like it was the ammeter case itself.
Stuart
