looking at the way it wires up
should I connect the small black wire on the first pigtail to the brown wire for the headlight signal?
Doesn't look right to me. Don't have a diagram handy, but see no reference to a brown wire in your first post. Brown on the stock r/r is a "monitor" (for charge voltage) and not used on a MOSFET setup.
I'm thinking the headlight trigger jumps from one of the yellow stator wires, to maybe... a blue wire? I would have to check mine, but can't at the moment. Best to trace it out on a wiring diagram, follow back from the headlight relay.
The monitor wire won't work for the headlight trigger since it is 12 DC volts, and the stator wires are 30-50 AC volts.