Based on my research, like Migizi indicated, there are 2 wiring harnesses. The door harness, and the mirror harness.
The bigger of the two is the door harness. If that doesn't have the extra plug(s) for the auto-dimming feature, then you have to buy and replace the door harness. This might not be a huge deal, but this might also mean that the wires in the main harness in the vehicle (that the door harness connects too) might not be there as well, and it is all mute at that point. One would have to take the door panel off to see if they have the plugs or not. Watching some youtube video's of initially delivered Model Y's kind of showed the wiring harness did not have the plugs. (see DAERIK's youtube video of his MSM Model Y that they got in March, he takes a door panel off.)
The mirror harness isn't as big of a deal, as if you don't have it, you would just get it with a replacement mirror unit. It would mean you can't just swap out the glass, but instead the whole mirror assembly.
DAERIK (youtube) got a quote from Tesla for both mirror units, and it was approx. $1.5k for the units themselves, which I think is a tad ridiculous imho. He decided not to move forward with the idea at this time. (though I would have loved to watch that video and see if it works.)
My Model Y was built a month (end of September) or so before they started including the auto-dimming feature (start of Nov. I believe), so I am "hoping" that maybe they started using the same door wiring harness and all I would need is new mirror units. (my vin is in the upper 52k range).