If you unmount one of your seats to install an amplifier, that is your best bet. Just make sure that you leave room for cooling (don't get a 4 inch tall amp). You will have to unmount the seat to secure it properly. I suggest thinking ahead when placing it. When you put it all back together, you should have room for cooling and be able to access the ports and controls from the rear of the seat.
The other option that I have seen is to mount it in glovebox, but that gets hot and is very small (it was a Directed 200W digital amp).
As far as the speakers go - I would contact Mr. Marv.