Help,fitting rear dash cam to reverse light circuit

Has anyone fitted front and back dash cams to a Skoda Octavia, the front dash cam is fairly simple as the fuse board is quite accessible.The rear dash cam has to pick up the reverse light electrical live feed to activate camera on selection of reverse gear, the feed cable is fed from the foreward cam so a cable has to be ran from the front windscreen, the problem occurs getting the cable inside the tail gate as well as the light cluster findig a route to conceal all the cables is the issue. The cam is waterproof I would like to connect above the number plate. 

If anyone has fitted cams and would like to pass on suggestions I would be most grateful.