In article om,
says...
Any ideas? Is a car computer my best bet? I've thought a small amount
about it, but draw a blank on the DAB and the use of steering wheel
controls.

DAB will be a problem, but you can usually use some types of steering
wheel controls if you can get some one to wire up a serial port adaptor
(some have been done) and use a program called GIRDER to translate the
button presses into software controls for particular applications.