#1 reason for frying the wires leading to the ammeter is the connections at the firewall bulkhead from the alternator and/or the battery not being weather tight and corroding over time. Corrosion = resistance = heat. Keep those clean and greased, and the connections tight at the ammeter and you should not have a problem. That circuit supplies juice to the whole car, except for the starter motor main circuit - either from the battery when the motor isn't running, or the alternator back to the battery when it is. The factory even bypassed the harness connection for that circuit on some fleet models and made direct connections for it, so it's a known issue.