I am interested in installing a security camera in my car, but I am worried about an adversary gaining physical access to the camera, and destroying the footage; leaving me with no evidence. Unfortunately, I can't just lock the security camera DVR in a room, like I could with a building. Uploading the data to a server would work, but if there's no service (or IMSI catchers, jammers, etc.) my footage is gone. So, how would I go about protecting the footage from an adversary with physical access, while optimally not using the Internet? I intend to use a Raspberry Pi with the camera, but solutions don't need to match my hardware and OS.
Are you sure physical access to the camera is your biggest concern here? I mean, if a thief breaks into your car, are you more worried he will see the security camera (and breaks it) or that your complete car will be gone? Besides using RF technologies (cellular networks being the most easy, common, and cheap) there is no way of securing the camera footage externally.
I would go for a more common GPS tracker, which activated by movement (geofencing), and reports back its coordinates through SMS. You can buy these for just 30 or so dollars, and can be easily installed in such a way no one would notice (especially not by a regular thief who's in a hurry to move a stolen car).
I think that your best option here is to have the entire setup hidden, the camera, the Pi and any other devices you add on. Hiding it will help keep it from being tampered with and the storage intact without trying to worry about a data signal wherever you park.
You could have a secondary hidden raspberry pi connected to a drive in the car. Then send the data through a wireless connection from one raspberry pi to the other. However, once they have physical access, its game over as they can do anything to the car and the items inside.