[Bill Harris]

If you wire them in series your going to have 24V and probably a fire somewhere. They need to be in parallel. If you have a battery in the trunk you have to have a master cutoff switch in the positive side of the circuit at the rear of the car so that means two high current cables between the front and rear, one for positive-positive between the batteries and to the switch, and another from the switch to the rest of the electrical system. Negative (aka "ground" side can use the chassis as the conductor on both batteries. It is a it of work but probably worth it in the long run. You can always remove the rear battery to lose weight if needed. Note that both batteries are supposed to be the same size and weight. I seem to recall some disqualifications recently for having a fake (hollow) battery in the front.