I just added a second battery to my Civic and i used a 8 guage wire for the positive and a 10 guage for the negative. I think if you use a 8 gauge for both you wont be doing any harm. to relace them take the battery out and follow the lines to their origin. The positive connects to the fuse box in the engine compartment and I believe to the starter and the negative connects to the chasis. It's easy so don't be intimidated.