It definitely wouldn't be illegal to remove a vehicle's cell modem from your own car in the US. The US is very friendly to vehicle modifications compared to much of the world. At a federal level, pretty much the only thing you cannot do to your own vehicle is remove emission equipment. The US does not even have universal requirements for insurance or safety inspections. And even in the states that do have safety inspections, there are typically very few pieces of safety equipment that are required.