I bought one from eBay called:
“ Bluetooth Music Handsfree CD Changer Adapter for BMW E39 E83 E53 E85 Business CD”
It just plugs into the back of the unit and my iPhone music plays when I start the car and stops when I turn off the car. Plus the skip to the next/previous track buttons on the Business CD work.
Btw, who in the world would name something “Business CD”? Dumbest name in the world award winner for many years in a row.