Unless there is some strange electronic control involved...
The light is a simple circuit, one terminal of the lamp is powered hot from the ignition switch. The other terminal goes to the regulator or alternator, depending on the system used. Look on the alternator for a name, like Autolite, Motorola, etc. Search "xxxx alternator wiring" and you can get a basic diagram of what's in there.
When the ignition is on, and the alternator is charging properly, both terminals of the light are powered equal, so the lamp goes off.
If the alternator is not charging, the power is not being returned to the lamp, so it would be on.
BUT, if the power from the ignition switch was not being supplied (as in a loose connection) the power from the alternator would feed back through the lamp and cause it to glow. If this is what's happening, look for corroded connections, especially if there is a plug together connection that is exposed to rain or near the battery. That lamp is also essential for proper charging, it "excites" the alternator to get it charging. If the circuit is not there, the alternator won't charge properly.
Usually the circuit that supplies the lamp will power other instrument related lamps and gauges. If other problems are showing up, they could be related.
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