Some new cars seem to have provision for adding aftermarket satellite radio, among other things. For example, the Acura TSX, virtually the same as the European Honda Accord, has a DIN-and-a-half sized storage pocket below the stereo. Honda makes a replacement for the storage pocket that can mount a single-DIN stereo and leaves a small storage slot. With that plus
a $100 3rd party adapter, you can fit an empeg quite easily. At that point, you're still using the factory head unit, which may not be optimal, but it's much cheaper than trying to do everything from scratch.
Of course, all of this varies from manufacturer to manufacturer and from car to car. I'm still waiting to hear how successful Bitt ends up doing the empeg integration with his Volvo.