i have not installed an extra battery to my forenza yet, but i had one on my 04 yukon. just find a mount, a mounting location, and enough 4 to 0 guage battery wire/cable. then connect the two batteries in parallel ( positive of one batt to the positive of the other batt, and neg of one to the neg on the other). the cold cranking amperage (cca) rating on the battery tells you the amperage of the battery at (if i remember correctly) 32 degrees F. by putting the two batteries in parallel you are combining the cca of both batteries and keeping the voltage the same (12V). but if you put the batteries in series, you have doubled you voltage (from 12V to 24V), and also you have to add the cca of each batt together to get the total cca. if you do put the batteries in series on accident, and turn the key, you can say good bye to your electronics and electrical system.
sorry for being wordy but its the only way to explain it correctly.
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