Where can I tap into a 12v power source on our 2014 cooper S to illuminate constantly an auxiliary light as I'm relocating the rear license plate?

I'm looking for something that is not going to trigger a light bulb error or something like that.

Most important, it needs to be on when ever the car is on.

I'd like to keep it simple so anything under the car, behind the bumper, or near the rear lights would be best.

Any details would be great!

Here is why:

 

Sorry, not familiar with the convertible. Is there a power outlet in the trunk? If so, that would provide 12v anytime the ignition is on.

 

Despite the appearance, for the most part I believe the systems are the same as an R56 since there isn't a whole lot of difference in functionality except this case for the third brake light and license plate which are part of a very different trunk door.

Yes, there is a 12V power outlet in the trunk. If I don't find something more accessible and dedicated to lights via under the car or behind the bumper that's what I'll do I think...

I don't think the 12v power outlet is the most regulated but it might just work fine for this. I think it shut's off too when starting the car but that shouldn't be a big deal. I'd have to access behind that panel and route the wire all the way to under the car, along the my trailer hitch mount, and through it.

 

So maybe a different question. What's the simplest way to go from behind the trunk power outlet to under the car by the exhaust area (It's an S=middle exhaust)?

 

So I did a bit of search around and I was able to find an access between the trunk and the underside of the car. I dug up the information from a trailer wiring harness installation video, I think from eTrailer.

Apparently there's a grommet that can be punctured for wiring pass through.
I checked and sure enough it was there, on the top left side of the compartment for the spare tire jack. It's all painted so it wasn't obvious that's what it was.

This will let me tap in the 12V power outlet.

Otherwise I didn't find any other way, short of going through the trunk ventilation panels which I wasn't crazy about the idea...