Hi.
My wiring runs across in front of the driver - along the "lip" on the lower panel above your shins. Then drops down behind the cover with the bonnet release in it and all the way along the floor, under the carpet to the rear seats. There's a big set of original loom for it to follow. Then up under the bolster and along the top of the wheel arch and down into the space behind the rear light. Remember that mine is in a Tourer.
There's nothing to stop you going on the other side of the car from the passenger fuse box along under the carpet and up past the rear seats. Or even above the headlining.
I fitted a camera on a diesel saloon a few years ago and fitted the camera itself by the number plate. This cable I remember being a bit of a pig to do as it needs to go out from behind the rear seats through a grommet and into the boot lid itself. IIRC it came out into the "channel" under the rear window and then into the boot lid. And you need to feed it "backwards" from the number plate location as obviously the camera won't go through any grommet holes. Can't remember where I picked up the 12V power from though. Might have been from the fuse box?
Hope that helps.
Good Luck.