tac0meat,
well, I had to mount/place the MTX amp under the rear passenger seat. THe space behind the cargo panel is real... picky?...
I built this real nice wooden mount for the amp, had it all screwed all and everything. Placed it (foam base) ontop of the wheel well and tried to put the panel back on.. Turns out the cargo cover sits like 3 inches over the well. SO no amp above the wheel well... wouldn't fit, cargo panel wouldn't go back on correctly.
THe other option was to mount the amp sideways like the factory amp. I didn't want to do this because it seemed sketchy to have only two screws in for the amp, so that it would be hanging from the metal under the window. Also, the factory wiring isn't the longest either. I would have had to spice some wire to make the connections. ANother thing that came up was the cargo side cabinet(some trucks have the sub). That got in the way if you mounted the amp sideways and too far back. I believe that the larger power amps, the bigger they are going to be than the factory. I wouldn't recommend putting the amp in the orginal place because it's trutly a bad position. Even if you did put the two screws in, mount sideways, it 'might' bang around. Also i didn't even try putting the cargo panel back on with it in place. THe weight of the amp and two screws thing irk me.
Another thing about putting it in the cargo panel. What if you wanted to tweak your amp? You would have to take the whole thing apart again! I'm getting speakers shortly and wanted access to the amp to tweak the music.
Under the seat was the best bet. THe weight of the amp keeps it in place and it doesn't move during driving. I didn't screw through the carpet, just let it sit there! WIring fits great, excess can be put next to the jack and under the carpet. Can't see the amp from outside (under the seat/ tinted windows). Can access gain/passes/wires/connections easily. AIr circulation is great, i like my music clean and loud. So i *CAN* feel slight heat from the fins of the amp during extensive playing. Also, you can put the seats down, doesn't interfere/crush the amp although it might hinder air circuilation a little.
hope this helps, let me know what you plan on doing or if you have problems.
-pete