I decided to do a quick google search and the sources states between a dozen-several hundred engineers are on the project. There have also been meetings between Apple and car manufactors.
It could perhaps be that Apple is trying to reinvent the wheel, but I doubt that they want to make the car completely by themselves. Sure, they have the money, but buying a plant and setting everything up is still a huge (HUGE) risk. Perhaps a iMercedes?
That's where I'd put my money on. Cell phones are seemingly a sure bet to be at the central hub of the mystical 'Internet of Things'. The fact that legislation is keeping your phone out of reach in the car makes it one of the few places where people have some 'distance' from their phone. This is unacceptable for Apple!
Anyways, if you were Apple, looking to get a further play in the IoT market, why not go for the most expensive thing besides a home? Home automation stuff seems to be a bit of a risky play, because the pace of change is too high. My parents house fingerprint deadbolts from a big company on their doors about 7 years ago. They don't work anymore, never really did - they won't be gambling on new tech anytime soon. I've read a few things about these Home Automation things fading out of relevance as fast they appear.
Assuming that home ownership is in decline (in urban NA anyway, renting making the most economic sense in these times) people just aren't investing enough into their rented apartments. But a brain in every manufacturers car, which could be an expectation within a few years (if not already) that plays nice with your phone, that might just be something Automakers would willingly let Neue Big Brother handle.