i worked on one and if you want the seats to be able to fold up and down you wont be able to get a sub in there. i only found 1 amp that would fit under the seat so i dont think your going to get a sub there.
smoker the only amp i could get to fit under there was a hifonics eagle so if he wants an amp youll have to get one like that or that size. its the smallest 4channel amp i could find.