
It's scary that there isn't anything safe available.
I had a car harness for my dog when I had just the one, as a non-driver I needed it if we went out in a friend's car. At first I had the attachment that clips to the harness then clips into the belt socket at seat level... but this was useless as he moved around and sometimes put his paw on the red button, releasing himself.
The other option is to attach the harness to the seatbelt without any plugging in... not great, although of course the seatbelts will freeze during braking.
Hobson's choice at the moment, having just bought my own car. The hatchback boot area is ok for one dog, but I can't find a crate or tailgate which fits and can't afford a bespoke one. I have installed a dog guard though... so that's one dog almost catered for., although she won't be very protected from the back.
For the other dog my choices are more limited, it's not a good idea to have the two dogs loose together (even if I had room). I've bought a Kurgo Zip Line which runs between the seatbelts on the back seat and has a strong leash to clip to it. Best of a bad lot of choices, and I'm not fooling myself it will be any good in case of an accident.
However - attached somehow has got to be better than unattached at all, if just from the distraction point of view. Neither me as a passenger, nor my friend as the driver, appreciated it when Dog 1 stepped on the seatbelt release button and then decided to sit on my lap!
Plan is to stick with this for now, until I can upgrade to a car which will take two crates securely fastened to the floor, and/or a tailgate.