Also, the CD changer input for the stereo ... add-on hardware, he built his own circuit board around an ATmega168 microcontroller and Bluegiga WT32 Bluetooth module. To connect to the car ...

You can even change the whole stereo head unit to a new one ... it’s one of the cheapest ways to add a Bluetooth ...

A simple Bluetooth module designed for wiring into car audio systems ... However, it requires adding in a Arduino or similar that can speak I2C, a DPDT relay, and some supporting components.