I decided to post a quick how-too on installing an aftermarket subwoofer in a G37. I have a 2009 sedan, but I don't think the coupes are much different.
First you'll need a few things for the install:
Sub and wiring kit
mechanical fingers (really makes life easier)
old wire hanger
First we’ll need to remove some interior panels. Pull the glove box, the passenger side, lower interior molding and the side panels (by the door sills), rear seat bottom, and the trunk cover mats. Next, let’s run the power wire. I recommend you start from the battery and run back to the trunk. The best way I found to run the power is to run it through the firewall on the passenger side. You need to remove the windshield wiper plastic and the battery. It comes off pretty easy; just don’t lose the plastic fasteners.
There is wiring harness grommet that we’ll use to go through the firewall. It's right behind the battery. Punch through with the wire hanger taped with the power wire...have the sharp hanger first then the wire (it helps to have a friend). It will pop up behind the glove box. (If you need to change your in-cabin filter, now is a great time to do it!). Once you have the wire, pull it through leaving enough slack for the battery connection (seal the grommet with silicon). Run the power wire down the side of the passenger area so you can put the panel from the side of the passenger floor area back on. Keep feeding the wire down the side of the car. The G’s have a terrific wire channel down the entire length of the car…all the way to the trunk! Getting past the middle of the doors will be easier with mechanical fingers. Keep going until you hit the back seat. There is a channel you can use to feed the wire into the trunk area. Depending on how much extra wire you have is how you go from here. (You can use the trunk pass-through as a shortcut)
Power is the biggest pain, but input wiring should be easy. For the Bose system, I tapped off the rear speakers to get a stereo input from the Bose amp in the trunk. Not 100% sure on the location for the non-bose system. You may have to tap off the head deck wires and run down the driver side (always a good idea to keep power and input lines separate). Make sure the aftermarket amp you’re using allows for “high-level” inputs, or you’ll need an aftermarket converter. The FSM will have wiring diagrams. For all the taps, make sure to solder and tape the connections. The crimp-type connectors suck and are not a good idea.
Here are the FSMs for all G’s: fsm-list-for-your-g-t525067.html
Hope this helps someone!