Are you using the Y cable that you had in the 2007 X5? Did your iPhone 4S work there?

If so, I assume it's the original 61120440645 cable (it'll have 0 440 645 on it). There are new cables like 61120440796 (which works with my X5 and iPhone 4S and 5) and even newer 61120440812. You can see the part numbers getting higher...