I finally got time to install my speakers yesterday. The MB Quarts sound great with the factory head unit.
The factory speakers are different than I've seen before. The mounting bracket is actually the speaker cage. The rear speakers are a little larger than 5 1/4" - more like 5 1/2" or 5 3/4".
To install the woofers, I just cut the speaker cone and the rear basket off the factory speaker and used it as a mounting bracket. The front was easy. I just used longer screws to attach the woofer to the factory bracket. For the rear I used construction adhesive and glued the MB Quart mounting bracket upside down on the factory bracket and screwed the new speakers to it.
The rear speakers I set up as coaxials and installed the front as components (photos below).
It looks and sounds great.