According to sources, this feature will work over AirPlay (With WiFI) as well as over USB. Additionally, Hamza Sood (who also found some hidden settings in iOS 7) has also found evidence that the iOS In The Car feature will work over WiFI.
Looks like iOS in the car works over Airplay (those preferences set values in com.apple.airplay)
Apple says iOS in the Car will launch in 2014 with several car makers.
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