this is some info on the 3rd brake light in a euro car. General conclusion: make a new wire as the loom is not equipped for a 3rd brake light in the euro version.
Take out the left rear light assembly, cut
the outer sheath back a bit and look for Black/Red wire. This is power supply to
brake light. Use a scotchlock connector or strip some sheathing back with a
sharp knife and solder a new wire onto the Black/Red. Poke a little hole in the
rubber grommet through which the lamp assembly securing post goes (the thing
with the big alloy nut on the inside)
Tape up your Black/Red wiring / solder joint, feed new wire through the hole in the rubber grommet (not the mounting hole or it will get worn through) and replace the lamp assembly.
Run the new wire up the left side of the tailgate opening under the rubber weather seal to the top right corner. You will see there is a plastic guide up there through which the wiring passes from under the headlining to the tailgate. The idea is to route your new wire through that with the existing, it's fiddly but I've done it a few times.
Remove the rear sun shades (four screws) then remove the top lining from the tailgate - the part with the interior light in it. It pops off but can be tricky, be patient and use a large flat blade screwdriver if required.
You now have access to your new wire and also the earth for the interior light (Black/White) and these are used for your third brake light. I use a strip of red LED type which has adhesive feet to stick on the very top of the rear screen.
This will not cause any type of bulb warning.
Hope it helps