Webmaster - excellent and SIMPLE device you've come up with here. I'm no EE but i can solder OK, and follow a simple schematic like this. I've decided to make mine a little more modular.
I have the remnants of a Dell L400 laptop. I say remnants, because it's one of those weird first attempts by a manufacturer to make a super-thin laptop, where the "docking station" is really a second layer that snaps into the bottom of the laptop, and is the only way to have a floppy or cdrom, or even more than just one USB port! It was given to me by a friend who knows I like to tinker, and it's missing that lower half w/ cdrom/fdd/etc, and has been dropped, so the plastics are in pretty rough shape. It's got a 30gb hdd and unfortunately only 256mb, but it runs XP, Winamp and Streets & Trips, so I'm ok with that. I found another thread on here with some great command line instructions on how to "pre" install xp onto the hd with the hd installed in my desktop. The only weird concequence of that was that now instead of C:, the laptop has an F: drive. Looks weird in explorer, but it works, so who cares!
I took off the screen (permanently - it's going on ebay!), and while I was in there, I probed around to find the power button stuff. While I was in there I also removed the modem, since I won't be needing it. A thought occurred to me at this point, because the phone jack actually had a 2-pin lead going from the motherboard up to the modem daughterboard, so I just grabbed a little length of cat-5 and took one pair out of it, and used that to extend the lead from the phone jack to the power button. This made "adding a plug" to the outside of the case super easy - especially since there was absolutely no room to add anything (further complicated by cracked case plastics!) So now other than the missing screen (and physical damage to one corner) the laptop looks normal - you can't tell it's been hacked! No idea what would happen if you actually plugged a phone line into the modem port now, but I haven't used a POTS landline in so long....
Now for the box I'm building off your plans, I am simply putting a duplex phone jack (cannibalized from an old 2-way phone jack splitter) into the side of the enclosure. I will have a simple coaxial power connector to plug into the car for 12v acc, but for the 5v section i took a rather different approach:
I am using the PS/2 KEYBOARD PORT for my +5vdc! Why? I only have ONE usb port on this laptop, and I am probably going to end up using that for the touchscreen, and I don't want to deal with usb hubs and their power issues. From another project, I had an unused female ps/2 port I can easily install into the radio shack project enclosure so that I don't lose my keyboard port in the process. For the lead to the laptop I chopped the end off an *OLD* keyboard adapter cable that allowed you to use an old AT style keyboard on a machine w/ a ps/2 port (See, there is a good reason to hang on to all those old cables and equipment - free carputer parts!). I'm simply tapping into the 5v rail on the keyboard line to activate the relay. I don't necessarily have to have a keyboard plugged in for this to work, though i will be using a numpad to control it - at least till I get a touchscreen installed.
The only thing I see as possibly being a problem is that I bought a 1000uf 35v cap instead of a 25v one like you specified. I couldn't find a 25v at any of my local radio shacks - 50v was more prevalent. Do you forsee this being a problem if I keep a 10k resistor?