I'd guess that the Heater on High, plus the coil, radio and gauges was just enough to go over 5A.
But the big thing is that the Jumper will provide power to the switch (and on to the Accessories), but the line from the Ammeter won't. That line will read voltage (12.2/3) when connected to its switch terminal, but turn the key and nothing comes to life (not even the fuel gauge). So looks like a bad line there.
