I was surprised too. I guess it's Porsches sales model to upsell as much as possible. Still didn't find a way to get my replacement mirror fitted though. Porsche dealer wouldn't do it because it's not original techquipment yada yada. The base trim on the Taycan is meh, and it's by design/marketing.
I believe the sensor for the outside mirrors is in the part where they connect to the body of the car, small rectangle. You can also see if a mirror is auto dimming under a certain angle the first outer mm of the mirror glass look slightly different shade. Thats how it is on my Taycan (and model S).
I got 971857511A from ebay, it came with a small plug that's coming out of the mirror. Unfortunately Porsche dealership in the Netherlands didn't want to fit it for me. Maybe ask your for a quote to see if it's something you can attempt yourself? If it's an expensive part and only little time it...