The 2003 has the battery in the center floor in the middle of the rear passenger footwell. I just did this for my 2006 G500 in order to run a fridge. The 2006 does have a 12v accessory plug in the left rear corner from the factory but it's not adequate for much.
You can run a 10 gauge wires with in-line fuse directly from the battery to either of the rear corners without too much trouble. The path runs under the carpet from the battery to the wheel arch just behind the rear seat. You can do this without removing the rear seat and by just popping up the door sill plate to release the edge of the carpet at the rear passenger footwell.
Once you have the wire to the bottom front of the wheel arch, you partially remove the wall panelling between the wheel arch and the door opening in order to feed the wire up to the top of the arch where you then can feed/push the wire under the panel that run atop the wheel arch. Continue pushing the underneath that panel until you reach the rear corner where you can remove the corner panel rather easily.
If you run to the left rear driver side corner, then you will find the sunroof motor located there. It takes up space but there is still enough room to fit one or two plugs (this is where the factory installed their plug). Another good option would be to use the area above the wheel arch to install the plug as there are square openings in the steel body panel. However this requires completely removing the covering above the arch which seemed more problematic to me.
Sorry for the long explanation. I didn't take any photos. That would have made it much simpler.
Another option is to use something like an ArkPak arkportablepower.com and not bother running any wires at all. Still need a way to recharge the extra battery though.