Ammeter Problems/Questions

" Light bulb plug melted " part is a puzzle to me. The amp gauge is mounted clear though the housing and not actually connected to the printed circuit board. I suppose the heat from the gauge and wires could transfer to surrounding parts.
You can use a 10-24 nut and bolt to lug the red and black wires together. The fusible link should have been the weakest link and burned open but it didn't. The weakest link was your amp gauge but it wasn't nessesarily the initial problem. I would check the bulkhead connections etc.. for a short circuit. I can't guarentee there is another fault that you haven't discovered yet but it is highly possible.