I fit an amp under both of my front seats in the Xterra. On the driver side I had to remove some piece of molded plastic over a wiring harness to get enough clearance. Just be careful when mounting the amp to the floor so you wont screw through any wires

Also make sure you can freely slide the seat back and forth without hitting the amp and route your wires so they dont get hung up anywhere...

NOTE: Obviously the size of the amp matters most here...Check dimensions before choosing an amp and you wont have to sacrifice space in the back.. and dont judge an amp by its physical size