Speaking as someone who worked for a corporate auto maker, it won't be an easy task since they try to make it as difficult as possible to disable online activation if even providing the ability at all.
The only real solution is pulling the head unit and trying to find any modem and desolder it, which who knows if it would function as it had before hand since everything is integrated.
When I worked on auto-maker on the head units, they were integrated on the chip, the ones that had a sim slot where you can insert and extract it were the ones for development. Recent cars, their GPS and screen media menus uses the Linux inside the modem chip.
Sounds a lot like a smartphone where one SoC chip does pretty much everything... CPU, cell modem, WiFi, USB host/device switching, quick charge, the whole lot 😢
Unless you get an expensive car, I think they do that to reduce expenses. Expensive cars have dedicated CPU for that, but they still communicate with the head unit for online data.
Or just put a power test attentuator on the antenna output.
It essentially absorbs the RF from the antenna and radiates it as heat. Since cell is pretty low power (1/2 watt max, IIRC), and a cell radio will stop trying to transmit after a while (though it will try again), I don't think it would cause any problems.
The newer cars from the company I worked for were always trying to phone home, not sure about other companies but this one was trying to lock you into the online ecosystem.