http://matthewbotos.com/maxima/wiring.html
This link shows how the guy ran it through a grommet in the firewall that is accessible by removing the wheel well cover on the drivers side. Turn the wheel so you can work in there. Take some of the screws out of the liner and pull it back. You will need something long and sharp to pierce the grommet, but there's nothing else in there. Pull the carpet back on the drivers side and feed the wire through.
I mounted my amps in the trunk and I found 2 bolts (one on each side of the trunk under the carpet, closer to the back seats than the license plate) that run clear down into the frame. I ground off the paint and cleaned up the bolt (rusty) and grounded it there.
I don't know anything about caps, never used/needed them.