I created an Arduino shield that plugs directly into 1996-2007 era Ford vehicles' nav/audio system and uses the original radio head unit controls to manage music playback on iPhone.
I wrote more extensively on the making of this project in a POST HERE that includes animated GIFs to demonstrate the features of the project.
I highly recommend skimming the post if you intend on making this for your own vehicle!
A demo video is available on YouTube here.
I also wired up my vehicle key fob to the Arduino shield so that the vehicle may be locked and unlocked over Bluetooth Low Energy. This part of the project is compatible with any vehicle with a remote key fob. BLE control is working, but is an in progress project.
The code, EAGLE PCB files, and BOM are available on GitHub at https://github.com/ansonl/FordACP-AUX.