Both the alternator and the starter are on that circuit so it could be either but I can’t figure out a way it would realistically be the starter.

I would have an alternator candy and possibly even a starter handy and then start taking it down but disconnect the cable between the starter and the alternator to determine which one of the devices is shorted. You can turn
Return the part that's not needed. If your parts store house boss and his conveniently close then you can just start the repair first determine what you need and go by that part.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)