I'm currently thinking along the same lines and here is what I've come up with.
take one of these wireless remotes (or something similar):
and hack into the buttons, rerouting them into some external push buttons that can be mounted to the steering wheel. I'm thinking of placing them on some sort of flexible backing, like a sheet of plastic, and then velcro taping them to the wheel. The remote itself would also be attached to the wheel.
let me know what you think.