Not sure if your question got answered, OP. I just did this with mine. No need to remove the brake light. Open the hatch and remove the center trim piece at the top of the rear window. Just give it a firm yank. The wires from the 3rd brake light are behind there.
And let me tell you how much it does grab attention. I have the flasher on all of my cars. It does not go unnoticed by people behind me. I get calls a lot, "is your brake light supposed to flash?" Yes. "why?" It grabbed your attention didn't it? It does a quick 3 flash, Similar to a firetruck or ambulance. As for it being annoying to the person behind you, oh well. But to be nice, mine has the capability to disable the flash for 10 seconds after hitting the brakes. So in stop and go traffic it doesn't flash every time I hit the brake.
tmarsh's post in 3rd Brake light 2015 Edge question was marked as the answer