Pair your Bluetooth-enabled device (e.g., smartphone) with the Bluetooth module on the car. Use a Bluetooth app or program to send commands to the car (e.g., 'F' to move forward, 'B' to move backward) ...

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 ...

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.