I think this whole open source and right to repair/modify will be very interesting with self-driving cars, because of their interaction with the commons. Here are some issues. There have been some discussions about the ethics of self-driving cars if it should sacrifice the lives of the people in the cars to save more lives. In a right to repair/modify wouldn't a lot of people pay to have the algorithm changed for the…
My feeling is that if there are regulations that require certain ethical behavior for self-driving cars, then there will also be periodic testing to make sure your car is in compliance. In the case of aggressive driving, we might also start to see traffic cameras doing automated detection of violations – if it sees that the gap between cars is less than 15 feet it'll take a picture of your license plate and send you a ticket (or even suspend your license until you can prove your car's software is in compliance).
But I suspect that on the whole people are going to stick with the defaults. The same sort of people who like to jailbreak their phones might mess with the software on their cars, but it will be an even smaller percentage of the population that engages in this behavior – after all, if you mess up you may not just brick your car, you might get yourself killed.