I purchased this when I first got my BMW and it worked great for $2.00.
http://www.amazon.com/MPI-Audio-Cabl...ref=pd_sim_e_1
That cable allowed me to charge the Iphone 4 and play music at the same time with 1 connection to the phone. However, once I upgraded to the Iphone 5 the cable was no longer usable with the new thunderbolt connection on the Iphone.