It's in!! I actually put it in!

Ok so it didn't exactly go off without a hitch but it's mostly sorted.
I wired it all up ok and was very careful to verify the voltages with a multi-meter. I then hooked the PC up to test it. Unfortunately I had connected the monitor power supply to the DVD drive. Ooops. Switched on the ignition and the drive crackled, ejected violently and started to smoke pretty bad! Fortunately I whipped it out quick and lobbed it out of the car so no major harm done but I do need a new DVD drive :(
Apart from that it booted up fine and the startup/shutdown controller works like a charm. The GPS mouse seems to be faulty but it was a freebie and I already suspected it wasn't working so it's no biggie.
I also have a lot of engine whine through the speakers but this is a problem I've had for a while which is caused by a ground loop and has just been exaccerbated by runnning even more power to the ground.

All in all it went as well as could be expected. The wires are nice and tidy, the P1290 controls the startup and shutdown nicely and the car diagnostics work perfectly. I am pleased :)