[strictlypersonl]

Much of what can be said, already has, but...

I would first check the ground at the voltage regulator and the firewall ground connection near the starter solenoid. If they are both good...

Remove the plug from the voltage regulator and (with the ignition on) check the voltage at the plug's green/red wire. It should read 12V.
Ground that terminal. The warning light should glow brightly.

If those things check out...

Replace the voltage regulator.