That is a nice idea.Just joined the forum today, when I see this post I thought I should share where I mount my FITCAMX Rear Camera.
Which I mounted right under factory rear camera, hope this helps.
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I didn’t have any problems mounting mine to the rear glass though; just left a little slack in the cable.
One thing though - in the wintertime rear glass gets super dirty around my area. There is some utility to mounting the camera in a location that the wiper reaches. Otherwise past a certain point the video is unusable.