Saw first on HN. It’s a very cool guide on how to do so, but more generally a write-up on why he did so and how to approach the problem.
Just keep in mind that as highlighted here (related thread (Rivian allows to disable data)) cars nowadays have emergency calling (calls emergency service in a car crash where you are unconscious) and removing the modem WILL disable that feature. That’s just a compromise you need to be willing to make.*
* Technically, some other solutions like a smart watch connected to a phone might fix it. But if you keep your phone in Airplane mode - as you should- I am not sure it would work,.especially on Graphene OS.