Crutchfield has some useful info.
http://www.crutchfield.com/Car/outfitmycar/car.aspx
"Door speakers
Behind the BRZ's door panels, you'll find 6-3/4", 2-ohm speakers. Removing and replacing them isn't especially difficult, but there are a couple of things you'll want to know before you start.
You'll need to remove the door panel, of course, and while this isn't hard to do, you will want to work carefully and patiently when you're prying the panel away from the door. There are ten clips that hold the door in place, and you don't want to break any of them.
You'd think a set of aftermarket 6-3/4" speakers would fit right in, but that's actually not the case. Better speakers require bigger magnets, and there just isn't room in the BRZ's door. You can install 5-1/4" or 6-1/2" speakers, but even then, you'll need to cut off the back of the plastic factory speaker grille to create enough space for the new speaker. You'll need a mounting bracket and a wiring harness to install the new speaker in the opening, and they're both included free with your Crutchfield speaker purchase."
http://www.crutchfield.com/g_400/All...rs.html?tp=105