If your battery is at 12.4V resting, then it’s no good. Charge the battery fully, then let it rest for 24 hours. The voltage should be around 12.8. The lower the resting voltage, the worse the battery’s condition, and the more it will tax your charging system.
My man, driving on public roads with only your e-brake to stop is reckless. It endangers other road users. I don’t care if this guy burns down his own car and subsequently catches his home on fire, all due to an electrical short, but it will suck if it goes up in flames in a parking lot and takes down some other innocent, poor person’s car.
I was once a struggling, working student driving an older car. I did what I needed to do to keep it running. I didn’t endanger others, however.
“My confirmation bias confirms that it worked out.”
This is no different than writing, “the first ten Google results for Sonoma flasher are…” It’s the most basic, non-contributing information that anyone would get with even cursory investigation. It’s fake-work that doesn’t contribute anything, but gives you the warm-and-fuzzies for “helping”, while most likely muddying the waters with misinformation.
I would bypass the resistor and replace it with a PWM motor controller. Resistors waste energy via heat, and you likely don’t want extra heat in the cab most of the time. As a bonus, you’ll have fine speed adjustment instead of just high/medium/low.
Check that each leg of the stator is good by checking that each yellow wire has about 0.3 - 0.4 ohms resistance against the other yellow wires, and that none of the yellow wires have continuity to ground. If one leg of your stator goes bad, it can overtax the remaining leg(s).
Corrosion causes resistance, which creates heat and lower voltage. Are just the connectors corroded, or are the wires also corroded?
To clean connectors, I use DeOxit. Spray it on, then plug and unplug the connector a few times. If that’s not enough, I use a soft, brass bristle brush. If that’s not enough to clean the connector to gleaming, then I replace the connector.
Incidentally, it’s common for motorcycles to barely charge at idle. You often don’t hit a 13.2 - 14.4V cutoff until a few thousand RPM.