Probably melted the main power wire in the steering column or in the bundle of wires entering/exiting the bulkhead connector, fusing incoming and outgoing power, bypassing the ignition switch. Been there done that.

When you repair it you should consider running a direct line from the battery to the alternator so the full electrical load doesn't have to travel into the interior and back again.

Even better is to ditch the ammeter and convert to a voltmeter which will also help reduce the load on the interior side.