I like to shut my engine off when I'm stopped for very long (I'm a bit of a hypermiler) and when I do so it shuts my stereo off. The police department who had the car before me installed a switch under the dash that allows them to turn the ignition switch off and have the car remain running. This works to prevent the stereo from shutting off when I kill the engine (flip switch on, turn ignition to "acc", then flip switch off to kill engine) but when I start the engine again, all of my electrical devices have power cut to them. My stereo has a built in MP3 player that reads files off of a USB drive. It takes about 20 seconds for it to boot up, read the files, and start playing again... Now I've come up with the below idea that would involve only one switch on my dash that I could turn on just before killing the engine, or even for passengers in the vehicle to turn on when they want to listen to the music and they don't have the key (say I've gone into a store or whatever). I figured that a secondary power wire run directly from the battery to the stereo, with a switch in the line for manual activation would be the easiest and cheapest. The problem with that would be that if I do not turn the switch off when I start the vehicle, power will run through that wire to all other electrical devices in the car that have their power cut when the cars starter is engaged. I figured that the solution would be a large diode installed in the factory stereo power line to prevent power from feeding back through that line to anything else. Will this work? If so, how large of a diode will I need to use?