It does have to have a SIM card somewhere in it, right? Can't you just pull that out? And if it refuses to work without one, put a SIM card that's disconnected or is from a foreign country.
Yes, I know, my current phone has one in it but I haven't used it yet, but should be handy for traveling (you can use it in dual-SIM mode by keeping the physical SIM card installed). This is a relatively new technology, and I keep seeing people say that the automotive industry really likes to stay behind on hardware. So I don't think it's likely they're putting these into cars just yet.
Trouble is the car has GPS too, so if it saw you were in the US with a non-US SIM card it might decide to insist you take it to a dealer. That's why I like the 50 ohm resistor idea.
Right, I tend to forget that most devices come with malware preinstalled these days.