Regarding the placement of the camera, I suggested earlier that a camera for rearward view should be placed just a few inches forward of the rear window, or directly above it.
You can find the optimal place to mount it with masking tape, then mount it once you're satisfied.
A rearward point on the roof is probably the best place you'll find, though, since it will allow you to see to either side of the vehicle in the other traffic lanes, as well.
A big advantage to cameras is the ability to record. If you have a hit and run, you've got evidence now.
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