Has anyone got any experience of adding an aftermarket subwoofer to this car (non B&O currently)?

I think there is already a sub in the roof of the boot space but i'd like to upgrade it. I was thinking of putting one in the space to the left of the boot behind a hatch in the boot lining. What would be involved, would I need an amp, and are there any existing power / audio connections I can tap into?

Thanks