I don't think a 2012 mirror will fit our 2013. The brackets and stem are different if you look closely. Even the mirror itself is different (the flashing red light is larger on the 2013). If it is a coding thing, that would be awesome.
As for testing your 2013 mirrors- put your hand behind the sensor of the rear view until it darkens. Then while holding your hand there, shine a flashlight into the front sensor. Your sides should darken. In any other vehicle with this feature, the sides will darken while only blocking the sensor (how my x3 loaner worked). For those experiencing this issue, take your cars to the dealer and have them file a report. The engineer said the more people bothered, the more likely a fix is released. I could take a short video showing you what I am talking about if you want.