3. Bluetooth-capable audio head units
This other method for adding Bluetooth to your car may be the best option, functionality and sound-wise, but it can also be the most expensive - replacing your whole audio head unit with a Bluetooth enabled one.
Although you can do this yourself with enough technical skills and the right tools, professional installation is recommended. Coupled with the price of the audio head itself, this option can get expensive.
The upside to this method is you can choose the features that you want and higher-end units can even integrate with your phone's music streaming apps, text messaging and hands-free functions.