I had a local audio shop install a powered sub in the trunk of my 2012 G37x Sedan, and the power wire was run almost exactly like mtrsprt's - through the body grommet in the firewall where the main wiring harnesses go through. However, somehow the installer managed to damage the grommet and now water is leaking through there and ultimately onto the cabin floor. I didn't know this until recently when the car wouldn't start because there was corrosion in the connectors on the wiring harnesses. But that's another long story. So, does anyone know if this grommet is replaceable without having to take it back to the shop and have them fix, or hack, it, or going to the dealership for repair? Thanks!

 

yup, that's how i did it. easy ... right near the battery.

 

1. If you unravel the plastic tape securing the wire loom and the large rubber grommet, you don't have to pierce the rubber at all. The rubber sleeve end over the existing loom can easily accommodate a single ought wire
2. There is already an existing wire raceway that runs along the seats and the door jambs on both sides of the vehicle. Run signal wires down one side and the power/ground wires down the second. There is no need to pull carpet or simply run wires under the seat. Both wire raceways terminate behind the pillar near the rear seat and behind the interior body panel

The actual writeup is from the G35 forums

2008 G35 Stereo install - Nissan Forum | Nissan Forums