I think you're probably right. I was reading some stuff on single wire (the fat charge one) alternators and I think people bodge them by just pulling a feed to the exciter direct from it though I guess you'd drain the battery after a while.
I measured the resistance through the the field coils on the alternator last night (D+ to ground) and it came in at about 800ohms so should draw around 9mA when the bulb blows and 14mA when the bulb is working so really small currents to get the thing working
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