I have done it using the Ipod Adapter on my Infiniti G35.
There are two buttons on my steering wheel I can use to control volume, skipping track
and swiching between several running applications on my computer.
There are several other buttons on the stereo unit that could also be mapped.
It is complicated and you need several pieces of hardware and software to be able to
make it work. You also need to know Apple Accessory Protocol and some understanding
of Lua Programming Language.
Parts I am using:
Ipod Serial to USB 3.3v adapter - $ 30.00 dollars at www.sparkfun.com
Girder version 5 - $100.00 dollars at www.promixis.com
Ipod Dock Extender - $25.00 dollars at www.radtech.us