Sound system For For Explorer 2004?

Alright i have a Ford Explorer year 2004, I love the space in it, specially the back to store my wetsuits and surf boards. and my seats need to go down so cant put anything under the seat. and i dont wanna risk alota space in the back of the car. i have a small compartment, if ur at all familiar with the explorer its in the back under the "floor" small though. i know nothing abut sound systems, i just want something in the back to make the b bump. The back of the car opens upwards i was thinking about putting some i really dont know the names of the things but drilling them to the side of that so when u lift the back it goes up to but sounds like alota wiring. whats a good size or product that i can get. i know vague but im hoping someone understands.