I'll certainly accept the possibility of bugs allowing remote takeover of vehicle cameras, but in general I don't think rear cameras are much of a risk to privacy. They're not always-on (generally they only turn on when the car is in reverse gear), and on many car models they're even physically covered when they're not active. (E.g. on my car, the manufacturer logo on the rear of the car flips up to allow the rear camera to "see" when I'm in reverse gear.)

Regarding bugs and remote takeover, the one thing that does feel like a mitigating factor is I expect for many car models it might be tough to find an exploit that enables the rear camera, but doesn't let the driver know that it's active. Usually when the rear camera is active, the infotainment display automatically switches to the rear view, and I wouldn't be surprised that there's no mode for "turn on rear camera but don't display the video feed to the driver" on most cars. Not as a security/privacy feature, but just because it's easier to write the code that links the two all the time rather than having it be conditional on something.

But yeah, as autonomous driving becomes more pervasive, there will be more cars with cameras recording everything around them at all times. I expect the manufacturers/operators of these cars to store that video for some time after capturing it, even just for quality control, bug tracking, and dispute resolution if there's a crash. And certainly law enforcement already has legal tools to compel the release of this kind of video.

I suppose this is just the next step of what's already the status quo, though. Many public places already have (stationary) cameras, either operated by the police or by private individuals, and police generally have the ability to access the latter, even. Cameras on autonomous vehicles just mean many more cameras over time, cameras that move around.

I don't really love this situation; I would like to see legislation that makes all of this data protected, and requires companies that store it to delete it after a relatively short amount of time (unless subject to a legal hold). But like most things, legal protections tend to lag behind technological progress.

My complaint (from above, not the only problem with cameras) about the rear camera is that the cheapest way for them to be installed nowadays, given that other people want infotainment systems, is for them to go hand-in-hand with an infotainment system. Whether the camera is on or not, the car is now vulnerable.

The sin being committed is having too much code, especially too much untrusted, untested code (it's "just a radio") wired into the canbus. Toss in a little auto-update functionality and some always-on antennas, and you have all the makings of a nasty exploit. Last I checked, none of those have been known to be exploited in the wild, but they've also not been patched in the last decade. It's a fundamental design flaw that saves a few dollars, so it persists.