(Don't know if this is any use, but drafted it the other night, but thought it best to read it through again in case of any errors, but just got round to it...
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Mr XX588, I'd guess what you suggest might not be possible, because I think rear-facing dashcams tend to have the lens mounted on the camera beside the main front-facing camera, thus it would be on the windscreen with the standard dashcam.
And that's certainly the way the two Nextbase rearcams work - it's just an additional unit that plugs into the main camera on your windscreen. It's the rearview and cabin view cameras along the top in the link below, which as the diagram suggests, just plug into the side of the standard dashcam itself (they look identical, but the focus is different - one is effectively a rear CCTV cam, while the other focuses on behind the vehicle and the field of view is a lot narrower than the cabin view camera, as can be seen from the illustrative photos:
https://nextbase.co.uk/accessories-parts/The third rear-facing option is the ball-shaped one, and that's intended to be mounted on the rear window facing behind the car, so doesn't catch anything inside the car at all
And problem with that option is that it requires a (supplied) wire from the rear window, and this plugs into the dashcam on the windscreen, thus obviously not nearly so easy to install compared to the rear plug-in modules. But I'm not sure if that option could possibly used as a rear cam behind a partition - it certainly has a wide field of view, but it's intended to be on the back window pointing outside the car.
But, come to think of it, I suppose either of the two plug-in modules could be wired from a plastic partition instead of plugging directly into the dashcam, assuming an appropriate wire and connection is available
I've actually got one of those ball-style Nextbase rear window cams lying in a cupboard.
That was the one I initially used, but didn't like it, because even the closest of tailgaters looked like they were miles behind the car, presumably because of the wide-angle lens.
So I got the rear-view camera instead (not the cabin view camera), which as well as giving a reasonable view out of the back window (although obviously a tiny field of view out of the back compared to the ball-cam thingy actually mounted on the rear screen), also catches back-seat passengers to a degree. (But doesn't catch front seat passengers at all - the cabin view camera would be required for that.)
And obviously the small plug-in module easier to install in my new Octavia, rather than the wiring to the rear window that I had on my Passat
Apart from that, it's all quite straightforward, though