Well, that sounds like it would work, but I didn't really want to get that involved with relays and such. I think i'm going to try the power supply option, but first I'm going to need to get the supplies to get the laptops working correctly. I'll probably buy the Carnetix 1900 dc-dc regulator from mp3car.com for only 100 dollars. If I can find an opus or something like it on ebay for a better price, I'll probably go with that. My factory radio has a nice volume attenuator with push button control. I can use it for the power and volume control (hopefully) with my computer system.
Now onto the lexusownersclub forum to see if my factory amps (pioneer subwoofer and speaker amps) will accept low level inputs (if not, I'm looking at even more to buy).