Its just as easy as adding a PS2, which it seems you already have... so to power them you need the inverter to supply 120 volts, or if you can find car power adapters for the selected systems, that works better. A switcher for the audio and video would be a good idea too so you dont have to switch cables to run a different source... here is a really cool switcher that mounts near the sources and uses a wired remote to switch sources. Or with this one, you can run different sources on each of three monitors...

I personally was thinking about putting my ancient Atari in my car when I complete a video system...