Alternator light / charge circuit questions - battery light is on but alternator good

[hoorah]

Okay, trying to wrap my head around this one. I know circuits decently well (had to take it twice in college....)

We have a late-ish 944 race car that recently started having the charge light come on. It hasn't been raced since this started happening, so this has only been while idling in the garage. It first came on solid, then the next time it was a very faint light, then the third time was something in between.

When running, I got +12.5V at the battery so assumed alternator was bad. Pulled the alternator and took to advance auto to test, they claim it tests good.

I did check the blue field wire, and I am getting +12 on that.

To slightly complicate things, since this is a race car, all power is routed through the battery cutoff switch (not the original wire for the alternator). I have confirmed continuity between the main alternator wire (power wire) and the battery - it gets 12V when the battery switch is in the "ON" position.

So I'm at a bit of a loss. It has a good connection on the +12, it has +12 on the field wire, and the alternator tests good at the parts store. The only thing I can think of is that there is a bad ground somewhere, but Im not sure how a bad ground would cause the battery light to illuminate. Wouldn't there still be +13V on the field wire if that were the case (giving me poor charging, but not necessarily an illuminated charge light?) Thats the one thing I'm not sure of.

My test test is to ground the alternator housing directly to the negative terminal, but after that, Im out of ideas (unless the auto parts store test is wrong).