There was no flange below the surface of the deck on my boat. I doubt if there is on yours. You might have to slip a hacksaw blade or coping saw blade under your connector to cut thru the wires. Once that is done, you can drill thru the deck as I described above.
(Justine, Your suggestion is similar to what I did when I initially added the deck light to the steaming light assembly. I later changed it to the way I described above when I found the wires leading from below into the deck connector were corroded.)
Good luck, Andy. Let us know if you have any other problems. Follow-up with a post when you have finished the job. I'll be curious to know how it turns out.