A FT-232 USB to serial board is used to program the Arduino. A standard Bluetooth module has to have HID firmware installed. [Dastardly] makes a homemade daughterboard–er, shield–to connect ...

In final form, a Bluetooth module and Arduino Pro Micro are stuffed inside the stereo along with a DPDT relay to switch the I2C bus. An external button is used to trigger the Arduino to switch to ...