In my (old) Clio I thought of putting it under the passenger seat but felt it would be a bit untidy. In the end I fitted it into a covered false floor that I built in the boot. Part of it is recessed to take the footprint of the sub box, the rest is about 4" deep to take the amp, some tools and space for another cd changer if I want one. I wired a computer case fan in there as well, which turns on when the amp switches on, and draws air through one side and blows it out the other.