Also, using control voltages for remote buttons is not terribly uncommon, either. Running a data bus up through the steering wheel adds cost and complexity. As a firmware engineer, I cringe at the thought of putting integrated circuits into something like a steering wheel. Not only would the cabling be a nasty point of failure, but the “disposability” (sure, they’re not disposable as such, but they’re an easily removed/broken/cut part) of the wheel also raises some hardware engineering red flags. A single wire, some buttons, and some fraction-of-a-penny resistors, on the other hand, seem prefect for such a location.

As for the audio thing, I do seem to recall several aftermarket kits that get between the CD changer and the head unit which basically inject your own audio in place of the CD. The theory is that you put a CD you don’t care about in the changer, plug in the iPod (or similar), tell the head unit to start playing the CD, then hit play on the iPod. Sure, it’s not an ideal solution, but the sound quality is certainly better than those FM transmitter things.

If you really want to get into the electrical engineering of things, it’s usually not too difficult [for someone who knows what they’re doing] to open up a component (amplifier or head), tap a few traces, and basically hack a line-in into the system.