i just have 1 question to everyone who has done this (im not critising anyone about this and i think you's have done quite well in your designs and builds) but from an electrical point of view (myself being an electrician) WTF COULD YOU POSSIBLY NEED IN UR CARS THAT WOULD REQUIRE 12 x 12V 30A FUSED OUTLETS? lol the FLC of that board would be 360A.
my suggestion would be to determine what you actually need in your car. wether or not it be gauges, extra cd/dvd players, gps power, UHF radios, etc. and build accordingly. yes i know that you will never fully load the board up (if u did you would need a cable to handle 360A - from memory a 95mm2 cable should do the trick + a decent size relay capable of taking 180A with a 12VDC coil).
for example, if you know that your going to have lets say 3 gauges, 1 dvd player and some neons/leds you build/fuse the board accordingly. a 3A fuse would be suitable for all the gauges 2gether, a 10A-15A fuse for ur dvd player (depending on type) and depending on how many neons/leds u have, 3A-5A should be enough. if you were to use say a 30A fuse on an unfused dvd player, if anything for whatever reason caused the dvd player to draw more current then required (less voltage than the required 12VDC will do this (ohms law says that voltage is directly proportional to current - voltage does down, current goes up)) and the dvd player was rated @ 12VDC/10A then the 30A fuse would probably never blow.
basically what im saying is that yes, a 30A fuse would blow during a short circuit, but if your equipment cannot take 30A then dont use 30A fuses.