There are two main solutions out there that I have found for the second generation Mini’s that will keep the steering wheel button functionality with newer iPhones with the lightning port. There is the official Mini/BMW lightning adapter cable that worked pretty well for me when I used it and the stereo and steering wheel functions work. The more popular option around seems to be the Bovee Tune2Air bluetooth audio adapter that keeps the same functionality as the official Mini adapter above but being cheaper and wireless.

Edit: /u/Royal_Ad_8143