The center speaker is for the navigation voice which makes the audio sound terrible, like it's right in your face. With the factory Ford audio system the entire thing sounds like crap. Being a drummer, I enjoy music as well as the "angry" exhaust.
I had the existing speakers replaced with FoCal, added a JL Audio subwoofer in the trunk area on the right side that is there from the factory, for that purpose and 2 hidden JL Audio amps. Powerful and quality enough to send the tweeters to meet in the middle to form proper center channel sound. The original center speaker is still there but can't be heard. I used the same head unit. The nav. voice goes throughout the full audio system.
Rear deck speakers are a funky size but there is a way to fix that issue. No extra holes or customization.