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.

Unlocking your car isn't exactly tough, but if you want to spice it up a bit, The Mercuino Project shows off how to unlock your doors with Bluetooth and an Arduino.

Walk out of range and the car automatically locks when the connection drops. His build uses an Arduino Mega with a BlueSMiRF Silver Bluetooth board that actively searches for his phone and ...