What's the best/easiest way to mount an amp to the back of the fold down rear seat? Can I just zap some screws in there? And if so what length screw?

 

just use some 1/4" to 1/2" screw..

easiest way in my opinion is to close the trunk, and outline where the baffle is on the inside (of the trunk side) with a chalk marker, so you know your amp will clear it when you fold it back.. then wipe off the mark when you're done.