Check the supported Bluetooth profiles for your car or Bluetooth device. Make sure the Bluetooth device or hands-free system in your car supports the same Bluetooth audio profile or profiles as your phone. To find out which Bluetooth profiles your car or device supports, check the documentation for it. After that, compare it to the Bluetooth profiles that Windows Phone supports.
Select the right audio source. For many hands-free systems in cars today, you need to select the correct audio source to play music over Bluetooth. Many times, the source will mention "Bluetooth streaming audio" or something along those lines. Check the manual for your car to get specific details and to learn how to select the correct audio source.
Check your data connection and date settings. When streaming music that's protected with digital rights management (DRM), make sure you have a working data connection and ensure that the date and time are set correctly on your phone.
See also
https://www.windowsphone.com/en-us/how-to/wp8/connectivity/bluetooth-faq
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