Flickering head lights, what am I missing

Mattax said
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.
View attachment 1715490127
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.

A test for that would be to measure votlage difference between alternator out and field terminal feed while the engine is running.

And more directly focused on the 18 gage wire, if you have enough wire for the leads.