Can I just run some screws into the back of the back seat? Is it wood under the carpet?

 

Metal. That is the front you are looking at. Where the foam would be and what people's backs would be against.
There is a fair amount of foam in there but still would leery of somebody getting stabbed in the back.
Worst case scenario and all.
How big is the amp?
The Infinity Amp that came in some higher end XJ was under the rear seat on the driver's side.
But those were kind of small. Guessing yours is too big for there?