What connection type is the steeringwheel controls? I was in my parents car today, and was wondering the same. If you can find some way to possibly connect it to your carputer via serial, you can use girder
http://girder.nl
to catch and do whatever you want with it (not just emulate keyboard keypresses, but change your volume directly). Girder forums are pretty cool, good place to gather resources.
[edited for accuracy -mugu)