Max I would ever expect out of a 12v cig socket is 5 amps. I know that technically they should handle 15 amps or so, but it's just not worth the risk as the majority of the time the wires running to the cig socket are pretty small gauge. What size inverter are you wanting to run? Even a small 750 watt inverter needs at least 4 gauge for even a short run since they can pull about 70 amps at full power, and up to around 150 amps surge.
And if you're going to go to the trouble of running cable to the back you might as well run at least 1/0 gauge for anything else you decide to power back there. You can just run one positive cable from the battery and then use shorter lengths for the negative and just bond them to the frame rails at the battery and at the rear. That will save on both cost and reduce the voltage drop (the frame has a very low resistance drop compared to any cable you'll run since there is so much mass to it, and the more the voltage drops along the run, the more amps your load(s) will need to pull to make up for it).
You can get arc welding cable relatively inexpensively from what I've heard. It's a good source for large gauge cable.
