I've downloaded a few rear view cameras from the workshop, but they seem to be intended for use as side mirrors and can't be scaled to realistically appear as if it is anything but a (very) long-stemmed camera. I did try taking a round mirror that had a straight (not angled) strut. By reducing the mirror diameter I could make it look as if it might be a rear-view camera (for viewing directly behind the car. When it's really tiny, it's hard to see that the "lens" is a mirror and not clear glass. If you like that idea but don't want to mount it on the trunk lid or roof, you could probably use the 3-dee controls to mount it so it pokes far enough from a back panel to look like the real thing.

You might want to dig around in the mods if you haven't got something that can be faked into something that looks the part. Good luck.

As far as rear mounted cameras go, I typically will use the "parking sensor" "single" from the misc fixtures category, painted with the "window glass (opaque)" texture. This, when sized and scaled appropriately, does an okay job of representing the lens of a backup camera. As far as hidden pop out cameras, you can easily use the fuel caps. I have a mod fixture under body sculpting called "headlight washer" which looks excellent as well for hidden cameras. I believe it comes with one of the seam kits in the workshop. Honestly, using the headlight washer and then placing the parking sensor over it looks pretty good for factory installed rear facing cameras.