This is an old thread but I just have to share where I'm locating my amp in my '05 Wave hatchback because I'm so excited about it! I placed in the styrofoam block that holds the tools in the spare tire compartment. Just had to cut out the shape of the amp and pop it in. I'm not worried about overheating because it is a class D amp and doesn't get to hot. I was all discouraged because this amp was too large to place under the seat. I was almost just going to mount it on the box but then this brainwave came. This solves many problems (it's totally hidden, makes it easy to take the sub out if I need the space, and doesn't take up any room in the car) I haven't completed the install yet but I can share pictures of the final install once it's done.
I hate to chime in - most of you know I did a bit more above and beyond for said "audio" systems - However I wanted to say there is a gromnet in the firewall next to the brake and gas peddle - this is a great place to run 0 gauge so it avoids heat and elements and easy access to the battery under the fuse box and under the windshield wiper fluid port.
However since your using 0 gauge... you can not fit it under the door jam/seal/trim. Now what I did originally was ran it under my center console along the gear shifter and e-brake. The back-end can bow slightly enough so its un-noticeable. then you can run it under the carpet along the "bump" of the frame and you won't know its there.
Or you can notch the plastic/cut holes in the frame.. I mean your choice. just depends if your running 1... 4... or 24 0 gauges (yes my friends 24)...to a copper block in the spare tire well