Here is my unique setup. I have a cigarette mistress. It has 2 on off switches. One is hooked up to the port motor and one is hooked up to the starboard motor. I have a button on the dash that when toggled combines the battery for that engine. I want to add one extra battery for a stereo. What is the cheapest way to go about doing this.

I already have a deep cycle battery ready to go

I know the electrical guys are lurking

If its only for stereo use and you dont want it to be able to be used for starting purposes...then just get a battery isolator and pick an engine to mount it to.....then the isolator will charge that stereo battery eveytime the motor runs....and you wont have to fool aorund with different switches or the factory wiring for starting....

I am assuming this will be used for Amps etc right?.....you still will be drawing juice from one of your original battery setup for the stereo unles you rewire that as well off the new battery.

Where can I get an isolator, are there any that are suggested for this application. Am I assuming that this will be hooked upi in between the positive side of the battery. This isolator allows the current to flow into the battery for charging but will not disipate both batteries. Only the battery with the amplifier hooked up will drain?

yup, i bought mine at autozone.