Does the light go out when you unplug the 2-pin connector at the back of the alternator? If not, you've got an unwanted ground between the light and the alternator. One leg of the 2-conductor circuit to the idiot light should be hot with the ignition on, and the other should ground through the alternator anytime charging current is not flowing. It sounds like you're getting your ground somewhere before the signal gets to the alternator.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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