Ok, had some time last week to do the rear doors as well as some of the interior panels, I took a few photos that I hope will help some people. I will also show how I mounted 6 1/4 in speakers in the rear doors. It was a pain, but doable.
First, how I mounted the speakers in the rear doors. I took the existing plastic mount, trimmed it flat and traced it on 0.22" MDF board that I picked up at Home Depot. This thickness was perfect, not too thick but still rigid.
Then I cut out the opening by tracing the mounting bracket supplied by Infinity. I have to say, that bracket was super helpful since it had hole alignments that fit perfectly for a Toyota. I did have to trim off some very long screw mount holes, but nothing terrible. I ended up putting screws straight through the bracket, MDF and into the door panel in the stock holes, except for one. If this is not an option for you, you'll have to cut out a second "ring" for a mounting bracket and then either glue it to the MDF board or sink the mounting screws so they don't stick up, and then put in more screws to hold the speaker to the ring.
Wiring was simple, no crossover to deal with.
The harder part was trimming the door panel. This does cut into the inside of the pocket, but there's extra space in there so you don't notice it. Even with taking 2" off the side there's no chance of anything falling inside the door. If you take off the panel you'll see what I mean.