If I bring my car for maintenance and we agree on checking brake fluid and air filters, they cannot decide "Oh, the profiles of the tires are off, we'll change them". No one would need to pay them for this, as it was not agreed upon and is therefore not part of a contract.
So your case makes no sense at all.
Except that perhaps you life in an insane country, or are not well versed in basic contract law :-)
I thing that the term is pretty self-explainable. Every support that the customer could request from the brand after the sale. Some of this things can be provided by anybody, other only by the brand.
But not all. I had never meet a single car owner that spend solid money in their new car and then decline the maker guarantee, but your definition of what is "normal people" may differ from mine.
Go touch some grass
They will not hesitate to change the Electronic Control module and maybe the BSI also, just to be sure that your car don't fails in the worse moment and kills you. This could start at 1500 euro or much more depending on the brand. They will not understand your "right" to run Doom in your car.
Think also that currently some secondary car systems in many brands can be deactivated online, without creating a situation of danger for you, but making your driving experience really miserable if they want.
"I didn't authorize that, I'm not paying for it, please give me my car back the way it was."
And if you think that, lets say "BMW", will hesitate a second to block online your car heating in the middle of winter; think again. I had seen it before. Even if you could hack it, to reconfigure the system again is not always easy or obvious without some knowledge of what values you need to enter.
It's not up to the car's maker to in any way interfere with or dictate the way I use my car.
I don't know why software would be any different.
It's more likely that you get it back with a new software (and all your modifications wiped), because as part of the routine maintenance some blue-collar technician connects a USB-dongle and blindly upgrades the firmware.
Unless of course, the modification is so popular that searching for it becomes part of the vendors routine maintenance protocol...
The idea of "Hacking" cars feels great until you realize that this cars are driving in the same road as you
Hobbyist car hackers is a group filled with overconfidence, and overconfidence can lead easily to a hell of pain and a million of ways to shoot yourself in the face. Because electronic sensors tend to be connected with other sensors, that are connected with many other unsuspected things, and those last things can be more important that it seems.