To stream music via bluetooth to the stereo, your car needs to support A2DP (advanced audio distribution profile) standard which is not as common as the HFP (hands-free profile) used for phone-calls.
Even though both A2DP and HFP is using the same bluetooth connection, the A2DP requires more computing power and different software to support that feature.
Car media systems developed 15 years ago (like for example Audi MMI 2G) where not ready for such technology as it was very uncommon to have mobile device holding music that you want to play wirelessly over bluetooth. With time the mobile device market got much bigger and the hardware required for the A2DP support got much cheaper and easier to implement in the media system software.
Right now most car makers are including bluetooth receiver that supports both HFP and A2DP as standard equipment. And even if it’s listed as option, most likely the hardware is in the car even if you didn’t pay for it. It’s only disabled with a configuration setting and can be enabled with a diagnostic interface (like in Audi MMI 3G and 3G+).
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