What would cause the 12gauge wire at the Junction box to ground?

[mr.precision]

I moved about 100 miles 3 months ago, and left my 1967 C10 in storage at the old location. Building a new shop at the new location.
Drove up there this week to start and drive the truck, which I do every month.
Dropped the battery cable onto the + post of the battery and went to start the motor. Noticed a lot of smoke coming from the area around the top of radiator before even turning the key. Jumped out of the cab and removed the battery cable.
Most of the 12 gauge red wire (12R in the diagram) going from the junction box to the main wire harness near the regulator was completely melted. Since I didn't have a tool kit or meter with me, it was kinda hard to diagnose. I'll be be going back this week with some tools to have a look.
The truck was just fine one month ago. The storage facility is at my old place. Totally sealed from the elements and from rodents.
So what would cause the main hot wire to ground? Can the alternator do this while sitting? The battery was disconnected while I was gone, so I consider myself lucky.
Ammeter? Ignition switch? Need some ideas here. Thanks.