DIY

Bluetooth range is too short. AM/FM frequencies are regulated so that individual cannot legally broadcast. So these probably aren't used to send data to manufacturers.

They're probably relying on existing channels that are IP capable, have good coverage, have relatively cheap antenna/MoDem available on the market. Such as 3/4/5G, Wi-Fi, LoRa.

Disabling those likely mean loosing features, ie emergency calls, anti-theft localization... But it may be the easiest way and can be an acceptable compromise for some. Flashing devices to keep using the hardware and features with more trustworthy software is potentially better. I fear regulators/vendors would put roadbloacks. All onboard systems may require some kind of certification to be street legal. If software certification is necessary, then hobbyist couldn't possible pay for that, which means in practice hacking you own device may not be legal. If there's an accident, except insurances to use any unauthorized car modification to put the blame on you and deny coverage.