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.

We’ve had very few car radios from scratch here at Hackaday, but we have had more than one bluetooth upgrade or addition of a line-in port. Thanks [Darkspr1te] for the story.

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.

Thanks to the LightBlue Bean+, however, one of those roadblocks has been torn down. The Bean+ is basically an Arduino board that can be programmed via Bluetooth LE.

Primo will include Bluetooth LE (low energy), NFC, Wi-Fi, and infrared built-in — you typically need a “shield” add-on to get these features.

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.