If you have an older car without Bluetooth built into it, it's an expensive upgrade to add. Redditor parkerlreed decided to create his own system using a Raspberry Pi.
The setup here does exactly what you'd expect. When the car is started, the Raspberry Pi boots up and automatically becomes a Bluetooth receiver. That connects to the car's auxiliary input, and then you can then play audio from your phone. It's a clever little workaround to add Bluetooth to almost any car as long as it has an auxiliary input. Head over to Reddit for the guide.