The ignition switch is not rated to handle more than the heater, dash lights and ignition. That is why the factory instruction for the radio call for connection before the switch. Your best option is to use a relay that is activated by the coil supply or ignition switch. Power the contacts from a fused supply.
The armature current is monitored and regulated by the regulator. The regulator would not see the current tapped directly from the armature and you could burn out the generator.
