I have used the main power feed that is on the inside of the car. There are 2 main cables that go to a connector. This can be found if you open your glove box, then take off the little panel to the left. In there is a fuse panel, and below and to the right are two large cables in a connector I think Blue & white from memory, with a yellow catch. I hope this helps. Unless the power draw is excessively high, these should be sufficient.
in the brazilian version, I run the power cable through the Firewall. As it is a Left Hand Model here , there is a round plastic cover in the firewall, just behind the brake pedal. I removed it, drilled to pass the cable, and sealed it.
probably the pic is not that good ... sorry.
it is the big black one. The small black is not a cover, is like a fixing clip.
The big one, if you look at the picture, you may notice the black wire (in fact that is a red wire but with protection cover) bending to the top left.
Hope yours is similar as that make really easy to pass the power cable to the cabin.
If you car is RHD, I'm not sure if it may have something similar in the driver or passenger side. Check both, just in case.