The right time was between 2010 (iPad launch) and 2014 (CarPlay launch), with a complete infotainment-only product.
Essentially, mimic the iPhone-on-one-carrier bargain.
Go to a struggling automaker (Fiat Chrysler?) and say "What if we told you that you won't have to worry about any of your infotainment solutions? We'll build the unit, in exchange for owning the exclusive app store it uses. And you'll get to say your cars are powered by Apple." Win/win.
Instead, they dicked around until the automakers figured their consoles out (mostly... still looking at you angrily, Nissan) and Apple was left without a key differentiator.
Hell, the mind-numbingly obvious reason for Apple -- do it at a loss for the real-time mapping and traffic data!!