On my 2012 sport with no spare wheel and an inner compartment for the side screens, you need to remove the rack (screwed into wood at the top), remove the blanking panel that sits above the compartment aperture, then remove the horizontal parcel shelf and then the side screen compartment. Then you can trace the wire from the existing third brake light, but it disappears down into the depths and you'll need to cut it. Cut it so that you leave about 6 inches on the 'feed' side to protrude into the parcel shelf area and fit the female connector. You'll probably need to insert a short length of wire onto the remaining tails from the existing lamp, so that the existing light can then also reach into the parcel area and has a male connector. The new light wire can then be trimmed with a male connector to reach the female connector. Leave a bit of slack in the wiring to allow for juggling all the bodywork back into place.
These are the feed wires for the existing light coming up from the depths
