Run 0-gauge (atleast one run will be good if you arent too high of power) from your starting battery's positive terminal to the rear battery's positive terminal and either find a good ground for the negative terminal of the battery, or run 1 (or more) runs of 0-gauge back to the front battery's negative terminal.
But the real answer would be to get a higher-output alternator, upgrade all of your wiring, and also add a battery (or batteries) to the rear depending on how high-powered your setup is, if its a smaller setup a HC600 would be a good choice...good luck