This project started because I wanted to mount my iPhone or tablet instead of buying a standard stereo.
I wanted to try building a project from scratch but I wanted to to be something I would actually use.
I started using the Arduino platform but I work on a Mac and kept having download issues.
At ARM tech con this year I met some of the (very enthusiastic) mbed folks and was persuaded to try the platform. This project is based on the Freescale FRDM-KL25Z board. I think it would work on any mbed platform though as the micro controller is only controlling the display (SPI in my example) and the Audio chip (TDA7419).
The ST TDA7419 is used in many car stereos. It has excellent audio clarity and provides the basic balance, fade, tone controls. It also has some more advanced features, the subwoofer cut off frequency can be changed for example. It can select from one of four audio inputs and can even mix one of the inputs into another one. This mixing feature would allow GPS directions to be mixed into the music from another input for example.
The data sheet has a good example circuit, it is in a package that is fairly easy to solder by hand as well if you want to wire this by hand.
For this project I decided to have a PCB made for it though because the tough environment electronics face inside a car. I had quite a few made (there was a minimum order quantity) so let me know if you would like one. The PCB includes the TDA7419, the reference circuit and a regulator to take the automotive 12 volts down to the 9 volts the TDA7419 requires.
You can see the details of my project on a Facebook page (here: https://www.facebook.com/ArduinoAudio). That is probably the best way to reach me if you would like a board.
You can see the library here: http://developer.mbed.org/users/danielashercohen/code/PreampTDA7419/ and an example program that uses an excellent Adafruit OLED display to show the current audio selection here http://developer.mbed.org/users/danielashercohen/code/car_stereo/
You can see a video of the project working here: http://youtu.be/8sRWzo-_Vn0