If you have source code then you can tell whether a particular executable was built from said source code. Pull the executable out of the car and also build the source code yourself as instructed by the manufacturer, compare the two binaries.
If the binaries don't match, then whatever certification the device needs automatically fails and it cannot be sold.
What that means is that later on, if "Something Bad" happens, you are in a position to be certain of what code was running. This makes investigation much easier as there is no chance that the original source code cannot be found when needed later. This does get a bit more complicated with software updates, especially OTA updates.