I'm thinking of buying a car. What is some advice? How do I maintain privacy in a modern car?

I'm sorry to say this, but I don't think you can really maintain privacy.

Sure, maybe you can disable some antennas to stop your location from being broadcast, but there are cameras in ever major road, so you cant get far without being spotted by a camera. Cars can also store data locally, then if you take your to the dealership, they will access the data. So you'd have to find an individual car mechanic that doesn't read this data, and at anytime, they can betray your trust. Not to mention, your phone is kinda already tracking you, so you'd have to turn that off as well, even dumbphones still have cellular triangulation and has even worse security. And whenever you need to use your phone, the carrier will know where you are.

So I think in the modern world with all these cameras, you kinda have to accept that you ain't gonna get much privacy.

Or the alterative is to not have a phone on you when you travel, take public transport, cover your face, etc.

If you want privacy, you'll have to sacrifice convienience.