High level repeaters?

Whatcha. We were talking today about replicating the lights and number plate up under the rear roof bar. The idea being that it's a level of redundancy for trailer lights etc not that I don't keep up to the maintenance but it's not without precedent that I have occasionally snagged the electric cable and not noticed until I've got back. So my question is where have people routed cables from inside to outside at the back without making a hole in the bodywork?  At the minute I have thought to run in the door jamb down to behind the bumper without actually crossing the seal? 

In through the osr lamp grommet?

Your options are fairly limited, existing grommets or cable exits are all you have. You can run a cable neatly inside the rear door jamb, considered this myself for a roof LED light. I was considering routing under the roof plastic jointing strips to hide it as I wanted the light at the front.
I eventually exited via the Ariel mount, but for you its at the wrong end unless you investigate the jointing strip further to route the cable.

How high do you want to go? Any merit in mounting inside next to the high level brake light? You could then follow this cable route if you can mount in a way to avoid internal glare. Just thinking outside the box.
 

Ideally I want to be under the line of my roof cross bars so I can still use them. It's unfortunate that to clear the roof with a legal size number plate would put the bar in the centre of the plate. I had considered internal mounted but because of the privacy gangster tinting it would be illegible.