Is your alternator the same Bosch used on the other geese? I know you've got the GM engine.
I just had an issue resolved that was almost the opposite, dash light would NOT go off even with the ignition off. Turned out to be a small piece of solder on a diode in the alternator that the shop TWICE missed.
Also check the ground to the gauge, if that's not good the light won't work. if it's good then I'd proceed with checking the other end of that wire to the light. is it black/white? should go to the voltage regulator if I'm not mistaken.
I've only got a few pictures of your goose (8MA670?) and none of them show the engine bay in detail.
MH
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