Very simple to do, I done it in my Fiesta.
I'v a piggy-back fuse connected to the fusebox, Basically it's like a double adpator. Replaces orignal fuse and has a holder on top for 2 fuses, 1 for the original from the car the other for the camera. Has a 12v-5v adaptor and then you can hide the wiring hidden under the A-Pillar/Headlining.
Comes on/off with ignition.
This is the one that I bought,
http://www.ebay.co.uk/itm/2915173773...%3AMEBIDX%3AIT