Sorry guys, I'm on a project of my own & just now got to your posts. That IS an extra connector for ANOTHER optional unit if you can imagine another one.
Isn't that a mess in there? You have to be very careful & get all those wires just right to get the head unit to go back all the way. Just put the connectors together that fit as my picture above and you'll end up with that one extra as shown above. If I can help let me know.