What are you saying? You trying to add 2nd battery as well? Look at Charlie's thread cause your question is very similar. If you like taking chances, you can just wire the 2 batteries in parallel and cross your fingers. I'd personally install an isolator/relay so that the starter battery doesn't get drained.
well i have a two batt setup. one is sealed in the back by my amp and a lead acid batt up front in the batt compartment. i drove it everyday for 2 weeks no porb. i jsut recently went about 5 ays w/o driving it and the batt was drained.
you can use 4 ga. from your bat under the hood to the battery in the back, because it is now just a charge line. it is more important to run your bigger line brom bat to bat or bat to amp. and isaac said the isolator is a good investment.