Car audio capabilities can be an interesting animal. Sami Pietikäinen’s vehicle in particular came with a built-in Bluetooth setup for hands-free phone use that does not support audio streaming. Fortunately, it did come with a 3.5mm line-in jack, meaning with a bit of hacking, streaming audio from his phone over Bluetooth would definitely be possible.
He chose a Raspberry Pi Zero W for streaming, and added a pHAT DAC for the audio output, soldering it to the bottom of the Pi to save space. Before he soldered the boards together, he bent out the debug serial port pins for access. He then included a clear acrylic case to make a rather nicely finished device that still shows off its hacker origins. Software-wise, he first used a Raspbian Buster Lite install, but eventually settled on Yocto to make a customized Linux distribution just for this project. It’s found on GitHub if you’d like to make your own audio streaming device.
Pietikäinen notes that a “smaller microcontroller could handle these tasks as well, but would probably require much more implementation effort for the firmware.” At ~$10 for the Pi Zdro W, it’s hard to argue with this approach, keeping a project simple means a greater chance of it actually getting done!