Running another battery for my stero.

I was wondering how to run a seperate battery in my car for my system. I dont have the cash to be spending on a new altenator so if anyone says to buy one then there's your answer. I just wanted to know how to run power to it? I take it I just run the power from the trunk battery to the power on the battery under the hood and same for the ground or just just do it to the chasis? Also when people relocate their batteries in their cars/trucks do they just spice into the harness that was connected to the power and run a wire from that to the new location?