By far not everyone modifying a car is a skilled mechanic. Many are, but by the same logic most people modifying car software will be skilled programmers.

>software modifications are pretty impossible to detect

I think there are two classes of software modifications: those that negatively impact the environment or other people and those that don't. The first is by its nature easy to detect from the behavior of the car, and I see no reason to regulate the second class.

Some people will introduce bugs, but I'm not convinced that's a problem that needs to be solved; self-preservation will make people careful not to introduce bugs in the break system, serious problems should be few and far in between