Flickering head lights, what am I missing

That alternator probably uses at least 3 or 4 amps when making a strong magnetic field. Could be more like 7 amps.
That's a long path for 7 amps through 18 gage wire coming from the battery in the trunk. Even longer when coming from the alternator.
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Arrows represent the current to power the rotor.
Blue wire to the regulator is mostly to measure the voltage and only takes a fraction of an amp to control the internal switching.