Very little outside of the in-dash infotainment system runs on Linux in a car.
I deal with manufacturers and OEMs putting software in cars for a living. I am not speculating, I am just reporting the reasons they tell me they will not accept any GPLv3 software for anything that gets loaded into their target devices.
1. They are lying, and spreading FUD about regulations and licenses as an excuse to hide the real reason why they don't want to let user install their software. (Eg, because options costs extra and if it was free software one could install it for free)
2. Or, they are mistaken and don't understand the license. Maybe the cost to use alternative is less than the cost of figuring out.
3. They are right and that is a sad reality that the government give more power to companies than power to end users for things they owe.
Since you mentioned something about the ROM which was clearly false, that could very well be option 1 or 2.
I mean, why would the gouvernement want to restrict users to update the GPS software or the media player?
then you do not need to make it modifiable under GPLv3. It explicitly states that.
anyways, it's generally not, expect any Linux system in a car to run from flash memory.