Instead of an isolater, consider using a solinoid. You will have a voltage drop across the isolator, of about .7 volts, i.e., no where near the 14 volts you't amp is spec'd with.
The solinoid would only switch on when engine is running, or a switch. I would suggest a timer activated switch if that were the case.
The best setup might be the solinoid closing when the ignition is on + a timed switch.