Typically we don't have power in the tub (older models, not sure about 2015 onwards). The electrical connections usually run up near the left hand tail light (two 10mm bolts can be undone after dropping the tail gate and remove the light assembly). Yours might not be the same.
What you might consider doing is introducing a new wire (just one, it doesn't even need to be heavily rated) to the run into the tub to provide you with "accessories on" power.
If you pop the little tray at the top of the dash above the centre console you'll be able to access the two screws that hold the facia panel in place. Pull this forward and if necessary, unscrew the 4 screws that hold the stereo unit in place. Look for the red wire in the stereo wiring harness and test it for positive power when the accessories come on (mine works like this, but I am using an aftermarket head unit with ISO plugs). There's a yellow wire as well, but it should be "constant power", the red wire should only be active when the ignition key is turned to "ACC" or "ON".
Tap into this wire, and use a small fuse (1A would be more than enough) and run a cable through the firewall, down under the car and up into the tub. There's a channel in the front corner of the tub if you want to use it, but be aware that there's a sharp lip at the top which will chafe the wires (that's what the fuse is for). This wire will come on only when ACC is on, but you shouldn't draw much power from it.
However, you can connect this wire to pin 85 of a 12V relay. Connect pin 86 to the body (tub) or the negative of your 12V supply back there. Then, connect pin 30 to the positive of your 12V supply and pin 87 to your camera.