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For more information on this, take a look at:
iOS: Troubleshooting car stereo connections
If you are experiencing difficulties with the connection between your stereo and device, disconnect the device, and then power cycle the stereo and your iPhone, iPod touch, or iPad. When the Home screen reappears, reconnect your device. If you cannot power cycle the stereo, turn the car off and then back on to allow the stereo to power off.
Best of luck,