You're going to have to either:
Contact the manufacturers of many different types of cars, and ask about what embedded computer systems they're using, and get all the specifics of their cameras and what connections you have access to. Honestly, I have no idea how its going to work, good luck. I don't think they have APIs that they just give out.
Or, you can use your own hardware. But then you gotta convince people to buy your software AND your computer, AND hook up your computer and your cameras to their car. Very unlikely.