When I had an R53, I bought a cable on eBay that plugged into the back of the Boost radio instead of the CD changer and provided a 3.5mm input at the other end. Something like this, although mine didn't have Bluetooth:
https://www.ebay.co.uk/itm/For-BMW-E...oAAOSw1bJemTmh
You could also swap the head unit out for something more modern.