Parker Reed decided he wanted to allow his car stereo to play audio received over bluetooth. So, he hooked up a Raspberry Pi with a bluetooth dongle to his car’s cigarette lighter power socket (using a USB adapter) and altered various start-up files. Then, it was just a case of wiring in the 3.5mm socket on the Pi to a line-in on his car stereo. You can read how he did it over on Reddit.