I am assuming you have a 02 Stang. First, there is no room under the seat unless you cut off the cushion, then anyone who sits on that side of the seat will have a nice butt warmer. Second, Heat dissapation. There is none under the seat. You will cook up that amp nice and hot.
3 choices -
1. Rear Seat Delete
2. Mount amp onto the back of the seat. (coupe)
3. Mount amp on the side of the trunk. (vert, coupe also)