I finished the installation today of a Rockville 10" powered subwoofer. It was pretty simple but there were a lot of steps (and going back and forth to my workbench to get the tools) so it probably took 8 hours total to get the seat off, measure everything, get the floor up, get the 'radio' out, get the wiring in, get the subwoofer installed, and then put it all back together again. There are convenient wire channels under the floor that go to the battery and the dashboard.
I mounted the volume control knob next to the gear shifter, on the driver's side. I can up the bass more if the road is really noisy. Music sounds a lot better now!