+1, Happened to me a few times via the following scenarios:
-The car was playing via bluetooth and I connected it via the white cable.
-Car was playing via bluetooth, shut the car off (completely as if you're leaving and locking it), plug in cable, turn on the car
Both occasions, I just simply turn the car off then on again and it fixed itself. Both occasions, with the cable connected, music still plays through the phone despite Itunes saying "Accessory connected"
Weird, but a bug in the combox / bluetooth?