George;

AMP Indicator probably provides Field exitation (fused circuit?), while regulator enables it (by closing a connection (on low side) to chassis) allowing Field current to flow...at least that is how a lot of alt systems do it...if you verify this is the way yours is hooked up and supposed to work, you should be able to do a Reg bypass test by shorting the Field terminal on alt to chassis (manually doing what regulator does automatically as it monitors system voltage)...if system voltage monitored on a voltmeter then goes up as you do this, that means alt can put out and it pretty much verifies a Reg failure...

Good Hunting!