I have heard an install with the Rear-view mirror center channel also..and wasn't impressed. I don't think either of the center channels mentioned here can compete with the Focal speakers...I was just throwing it out there as an option since you will have that extra din location to play with.
But anyway, stop and think about the purpose of a center channel. In a theater, it takes the dialog and centers it on the screen where the actor's mouths are whether you are sitting to the far left, to the far right, or dead center. The people who sit in the center don't really need that center channel. The voices would be centered by placing equal volume on the left and right speakers. But without that channel the people on the sides would not be able to match up the actors mouths to the sound they are hearing.
Then there is your home theater. Usually your left and right speakers are set far apart and the seating is not usually optimal (unless you are the one sitting on the center cushon on the couch or you place an easy chair directly in front of the TV). The center channel again, pulls the voices of actors back to the screen which is small relative to the distance between the left and right speakers.
Now on to your car. Again the same principle applies. You have a wide sound stage but the screen is very small. Installing a large woofer on top of the dash will not help pull the sound to that screen but instead it will put the sound out on the hood somewhere. A din-sized speaker enclosure will do the job perfectly, but the SQ will be sacrificed. Really, the best solution is to use a phantom center channel setup. Let the speakers in the doors do the work. If you angle them properly the center stage will be about as close to the screen as you can get anyway. Maybe a small tweeter in the dash could compliment it but that is all.