can't cut steel
Caveman,
I went to Tweeter's (usually I avoid them because they are generally more expensive, but they weren't too bad $1000 for amp, 2 10"s, installation, and custom work). I talked with the installer (not the salesman). Originally he thought he could do the same thing...build a box and cut holes for the 10" inches right behind the seat, but after taking some parts off, he discovered the steel wall has structure ridges in it. He felt that if he cut through the steel I might have problems with the truck flexing and things like a cracked windshield could be the result.
What he is going to try this time is put the 8" subs in the same box as the tweeters are in (so they are not right behind the seat. Of course behind the seat was an option, but I only wanted that as a last resort). When you have the roof cover open (but the roof still up) and you look into the roof storage compartment, attached to the back of the seat wall is a plastic box with two latches. In this box is the factory amp, a computer module and the rear tweeters. This job is going to cost about $1100. Some other quotes I got were from $1500-$2500 and that was for the original plan. I'm also electing to put in some bass blockers on the 6 1/2"s in the door.