Yeah, Stuart is right on here, wire it so it reads correctly. But see, in my old under-developed brain this is exactly how it should be wired. This is how I see it:

Since the alternator, when running, is "more positive" than the battery the ammeter "+" should face the alternator, not the battery. But
the engineers at GM didn't see it that way so a stock ammeter is "+" towards the battery. That seems backwards to my way of thinking - pass the vodka Stuart.