[henrys57wagon]

You want to keep both batteries separate. The second battery would feed your stereo etc. You would wire in the isolator as you would wire in a relay( which does the same job as an isolator). A 12ga wire from the truck battery(alt would do) to the isolator then to the spare battery, (for recharging only) this is the part of the relay that is not energized when the key is off. Then a signal wire(16ga) from a run circuit to the coil of the relay then to a ground. This is the same as wiring a fan relay, hope this make sense, Henry. PS you want the batteries in parallel.