let me describe my project. I'm selling my current car, an s13 240sx, and buying an mr2 from a friend of mine. I plan to build and install a "carputer" into it, to take care of mp3/dvd/emulation/gps/EMS needs. It's been done before, many, many times, so I think I'll give it a try. Here's how I'm going to rig it all up:
The carputer will be a ATX pc setup using a mobile computer case and DC/DC ATX power supply from www.opussolutions.com with an atx motherboard, probably a 2.4 athlon xp (something cheap, my recurring theme), a ATi 9600 128mb card, and a turtle beach santacruiz audio card. I'll use some cheap pci ram, probably a gig (to handle the gps and ems), a USB2 keyboard/mouse combo, and a firewire dvd/cdrw drive. It'll also have a wireless NIC so that i can remote access my car when I'm at home. That'll be the "brain" of the system, and will control audio, gps, the engine management system (much later on) and also the dvd/emulation capabilities.
The subwoofers will be powered by my 400 watt 2 channel amp, bridged (as they are now) and the RCA leads will be converted to a standard adapter that lets them plug into the turtle beach. The component speaker's crossovers will be plugged into a 4 channel 350 watt amp, which then routs the rca lead to an adapter that plugs into the turtle beach, just like the subs. That way I'll be able to control my sound using the winamp equalizer. I won't have fade but who needs that ****.
For video/dvd capability I've got one of those 8 inch flatpanel car screens. it works really well. I'm going to rig that up to the ATi card using the RCA outs. It's going to be flushmounted into the center console where the current head unit and junk slot is. Below that, I'm going to mount an external firewire dvd/cdrw drive. For controlling it all I'll either have a USB2 mouse/keyboard combo, or get a small keyboard and somehow rig one of those trackball style mouses into the ashtray. we'll see about that.
The DC/DC power supply is directly powered by the car battery (like a cop car/ambulance) and can use an ignition pickup wire so that the system comes on when the car does. I think I'm going to leave the feature out though, because the mr2's got a pretty small battery and it would probably drain the system way too much on initial crank. I can just rig an on/off switch for it, once the car's cranked. I might switch to a larger deep cycle marine battery (like the optima blue top) if I notice my battery having a really hard time.
My favorite feature is going to be the wifi card though. It'll be pretty kickass to be able to remotely access my car from my house, so that I can upload music/movies/games/you name it. It'll also make tuning the ems and GPS maps allot easier too.
Besides the GPS/ems systems, the carputer itself should be relatively cheap and easy to setup. I've already got the subs, speakers, both amps, all of the wiring I'd ever need, and even half of the computer components. The only thing I really need to do is get the car (in a few weeks that'll be done) and get the rest of the components (namely build the computer). Installation will be pretty interesting, because I want the car to retain a somewhat "stock" look. the keyboard's going to be stealthed to the glovebox more than likely, so that when I need to use it I can just pull it out. If I can find a way to stealth a trackball mouse I will, but otherwise it'll probably be just a keyboard/mouse combo.
I'm really looking forward to setting up and building my "carputer" and will document the install for everyone's enjoyment. It shouldn't cost more than a few hundred bucks (the case and power supply will cost the most, by car). If anyone sees a problem/hole in what I'm planning let me know, as I want to make the system really nice.