I have the 12volt Coleman cooler. I wired another cig outlet in the passenger back area of my '97. I ran the wires (ran 4 so I have extras) under the carpet right behind the 2nd row seats to the driver's side. Then there is a channel that you can run wires in all the way up to the firewall...a nice grommet already there, made a slice in it and bingo the wires are in the engine bay.
I ran 4 solid #12 house-wiring wires (THHN or whatever, the single wires, not romex stuff). Running the solid wires actually made it alot easier since a bundle of 4 of those is very stiff, so you can push it where you can't see, much easier than using flexy wire.
I have don't use the body for ground, I have power and ground connected to the battery directly, a 20 amp fuse right at the battery for this outlet, and since it's on the battery, it's always on. This setup has worked really well for me. Only downside is now I want more than one 12volt outlet back there, I have all the stuff already there to wire another one, just haven't done it.
I run the Coleman cooler off that outlet, runs all the time, works very well.
Hardest part is securing the coleman cooler since it's really light and likes to bang around.
The actual cig outlet I used was from Walmart, they have a 'Truck' one designed for a generic mount, comes with like 20' of red wire and like 1' of black wire (they are assuming you're using the vehicle as the ground). You just drill a hole and this goes in, then a screwon back holds it there, cute little spring cover...I hacked both wires and used crimp connectors to my #12 wire I ran. I probably should have soldered them but didn't have my soldering iron at the time and soldering solid #12 copper is a pain.
Good Luck...
Mark Brodis