[msbhammer]

Nah, I wouldnt go with a third battery at all. I figure you have two batteries, one battery for starting and the other for lights, stero, ect. Thats plenty of battery, I guess you have an alternator to charge, but either way, you can run alot more stuff if you wanted to. You can run the stero straight off the battery with a inline fuse attached, or just run the stero off one of the fuses from the fuse box, wich ever is the shortest distance I figure.