Some car stereos require a firmware update for AVRCP 1.4 Bluetooth compatibility
iOS 6 or later supports AVRCP 1.4, the latest Bluetooth standard for control and playback. Your car stereo (sometimes called a head unit) may require a firmware update to correctly support the Bluetooth standard.
Several manufacturers, including Advent and Pioneer, have announced plans to provide an update or already have one available. Please contact your car stereo manufacturer to see if a firmware update is available.