LibRedirect has this functionality:

https://addons.mozilla.org/en-US/firefox/addon/libredirect/

If you’ve removed or disabled the telematics module and its antennas then your most sensitive data - your location - can’t be collected. GPS and mobile data technologies don’t work without hardware, antennas, and electricity.

At that point even if there’s a back-up collection system the most a dealer could dump would be general driving and usage data. That’s a non factor for 99.99% of people, but if that is an issue in your threat model then you should avoid dealers and work only with trusted, independent mechanics. And frankly if your average speed or odometer reading is that sensitive you’re probably on the run and have bigger issues to worry about.

I guess they could also dump your contacts or call data if you’ve synced those with your car, but you shouldn’t be doing that in the first place. Data collection isn’t magic. Don’t give the car data and it won’t have it.

Shop for cars that work fine with their telematics modules & antennas disabled or removed, disable/remove them when you buy yours, and you’ll be fine.

No, that isn’t likely. People have fully disassembled these cars. There isn’t a secret second telematics module inside the seat cushions. If you disconnect power from the telematics module it can’t transmit data. If you want to be extra sure you can also wrap the module in faraday material, disconnect the antennas, or remove it completely. Data transmission isn’t magic; it requires hardware.

At that point the most that could happen would be a mechanic dumping the data and uploading it to GM. Big corps are high resource, low motivation adversaries. They’re not going to spend tons of time and effort going after the <0.1% of people who physically disconnect telematics modules.

Depends on the car. IIRC in newer GM cars you can just pull the fuse for the cellular modem and generally just lose the connected features.