Pretty much all hidden and nothing done that can't be undone (bought aftermarket pillar moldings etc) There are LOT's of different ways you can go and lots of options. Like Secret audio etc... The Bass link or even a KENWOOD KSC-SW1 woox powerd sub is small and easy to add to any system. You can do the 6x9 in boxes in the back seat, store them on the floor, or go all out. Anything you can imagine.
Still working on mine and adding as one of my winter projects...