Hi
I fitted a front and rear dash cam was going to fit the cable under the mats but opted to run the cable along the roof lining just along the edge using a trim removal tool to lift plastic trims and tuck cable out of sight.
Then when reaching tailgate removed rear plastic trim cut a tiny piece out so cable wasn’t snagged when opening and closing boot fitted camera to rear glass been a year since fitting all working ok.
The problem you have is powering yours from the tailgate area mines powered and controlled from front camera which is the normal mine is a 4K one which you can access recordings without removing cameras straight on to your phone no problem either with the very dark tinted rear glass for recordings.
Let me know and I can put some pictures and model nos of what I installed and fitment if it helps.