ignition switch ???

Only thing I know at this point is to backtrack to the ignition switch and see what you have there. Check main power TO the switch with it "on" and see where you are losing the voltage. If it drags down the main power into the switch, then the problem is on back towards the battery............welded splice in the ammeter wire, ammeter connections, or again, bulkhead connector.

If the load was pulling it down that much things "should" be getting hot and smoky, smell of warm plastic, etc