I don't have the specific charge circuit for your year but usually the charge lamp is powered thru the ignition switch.
The alternator regulator controls the ground back to the charge lamp.
When the alternator comes up to operating level, the regulator senses this and remove ground from the charge lamp putting it out.
Since you have replaced 2 alternators and their built-in regulators, I have to feel there is a ground on your lamp lead outside the alternator plug up.
I would make a resistance measurement from the lamp lead to ground, off the alternator with the plug off the alternator and the ignition switch 'off' to see if there is a ground completeing the lamp circuit, on a hard ground basis and keeping the lamp lighted as long as the ignition switch is on and supplying power to the lamp from that end.
This would not really interfere with alternator or charging operations, otherwise.