[yrean]

your normal computers use a 110v PS which breaks down to 12v, 5v, and 3.3vdc. In a car you already have a 12v dc source,(your battery/charging system) so you use whats called a dc to dc powersupply. these normally incorporate a startup/shutdown controller as part of the PS so your ignition switch can control turning on and off the pc. hope that answers your question. not to mention your more then likely only going to pull a max of 20amps which is only slightly more power being drawn then a head unit