I would mount the amps and crossovers to the top of your subwoofer box, depending on the size of the box, and the size of the amps. I have my amp and a capacitor mounted on the top of my box, and there is plenty of room for another two. This is the best way to mount them as well, because the heat sinks work most effectively with the fins pointing up. Plus, you dont have to worry about the box sliding into them. I wouldnt mount the amps to the backs of the seats...they are pretty good insulators, ESPECIALLY if the box is placed, or slides, really close to them. My friend cooked 2 nice amps after not listening to me, and mounting them both on the backs of the seats. And you are smart not to mount them upside down under the rear deck, because then the heatsinks wont work very well at all.