I have my amp under the passenger seat. Somewhere I've actually posted a picture of the yellow airbag connector plug under the passenger seat near the transmission tunnel. It is a tricky little bugger to unplug. I'm pretty sure I even posted the manual picture and how to undo it. Will find it for you.
In terms of mounting, I got a sheet of wood shelf stuff I had lying around - maybe 12mm melamine or something like that. I screwed the amp down to that and stuck black duct tape over the visible bits of the white surface to make it dark and basically disappear in the shadow under the seat. At the front of the seat there is a step in the floor pan which will stop it moving forward. At the back I ran a small piece of hollow flat bar across the seat mounts. I must have fluked the size because it all ended up being pretty snug and doesn't move around at all. I haven't had any issues with heat. It seems to breathe at the front and the left hand size is pretty open. I've usually got the floor vents open on cold in the front anyway. In the back I have carpet mats that are supposed to go under the front seat. On that side I've sort of cable tied it back so that it goes upwards and kind of closes off the gap under the seat so you can't see the amp or the wiring and so you can't really kick it if you're a rear seat passenger.
Right, now this is where I posted about the plug:
http://www.newtriton.net/phpbb/viewtopic.php?f=23&t=4688&p=69704&hilit=mmcs+plug+play#p72217And it looks like this:
Looking back at that thread, I folded the back of the seat forward, and then tipped the whole seat backwards into the rear seat and then had enough room to work without having to remove the whole thing from the car.
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The two threads I wish people would use more:
thing 1 and
thing 2.