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1969 Pontiac GTO
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Remove the external regulator (or leave it in place if you want for appearances, but alter the connector so it's actually not doing anything). This link from a :cough spit: chevelle forum has the wiring connections you need at the former voltage regulator connector in order to make your GEN indicator light work, and also which terminals to use at the alternator.

Alternator Conversion

(Ignore the wire colors in that post and pay attention instead to which terminals are connected to which).

Buried in the text of that link is an explanation of where the alternator connecter terminals go:
"S", this was a heavy gauge, red wire.​
"F", this was a small gauge, brown wire.​
"L", this was a small gauge, brown/red wire.​
"P", this was a small gauge, brown/white wire.​
The red wire from "S" gets connected back to the output terminal of the alternator just like in the "SI" swap.​
The brown/red wire from "L" gets connected to what is shown in the diagram above as the blue wire. It's this wire that comes (indirectly)from the idiot light and it energizes the alternator (and also causes the light to turn off when the alternator is charging - Bear)
The wires from "F" and "P" are not used
So, at the (former) voltage regulator, connecting the "L" terminal from the alternator directly to the (brown in your wiring diagram) wire that goes to the "H" connection on the firewall bulkhead connecter completes the circuit for the indicator light.

Easy Peasy

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