Well, yes, but it's an implied license. The purchaser of a car doesn't sign a separate piece of paper permitting him to do stuff with the auto's code. It's like that video you bought over the weekend. Copyright law prohibits performances of protected works. Can you invite the neighbors to watch that video for free? Yes, because with your purchase you got the implied license to do so. Can you open your own movie theater and charge admission? No, because your implied license doesn't cover that.
Now, if you want to look at the code, copyright law won't stop you from doing that. (But the manufacturer might by not giving you a port of access.) If you possess the copy of the work, you get to look at it. You just don't get to reproduce it or perform it (say, in another model of a car, modified or not).
The manufacturer might claim the code to be a trade secret too, but that won't work very well because they will have published it by putting it in the cars they sell to the public.
So unless the car manufacturer is going to make the purchaser sign a contract not to fiddle with the code, and to make any subsequent purchaser bound to the same terms, I think they just have to put up with the modders and the rodders...