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.

The Bluetooth module wants 3.3v logic levels so that is taken into account. The image above shows a buffer chip doing the conversion, but the Fritzing schematic on the post uses a voltage divider.

The $22 kit includes an Arduino-BLE board and 6-pin header that should be fairly broadly applicable for building Arduino-based hardware that can communicate with iOS devices.

Your robot army, controlled by Arduino and Bluetooth Open-source hardware project shrinks Arduino, adds wireless, creates awesome.