Nice! I'm thinking about picking up a processor and going 3 way active with a center channel. The only thing stopping me is finding a place for the extra amp and processor. I still use my truck to haul stuff around, so putting a board behind the back seats is out of the question for now.
After a quick search, I found this guy upgrading his system.
MISC: 03 Toyota 4Runner Full Audio Makeover - AcuraZine Community
What you could do is, if you cant figure or find how to mount the speaker, go into a Toyota dealership and see if you can buy the bracket from them. Or go in there and take a gander at there parts manuel/brochure, to see where the bracket will go. The guys working at the parts counter are usually pretty cool about it, at all the dealerships i've been to.
If you do find a way to do it, with out the oem bracket, let us know.