Hi dude, u hit the same dilema I did when doing mine, totally ignore the fuse box thing it's just a great way to over complicate things lol, as above pull out the little tray below the light switch and also the fuse box cover as this makes it easier to see the back of the light switch, one of the wires is a live wire and you can splice your boost gauge power wire in to this! I can't remember what colour it was now I think I got it right first time as a blind guess lol, and I used a screw in the fuse box somewhere as a ground as there is one that goes straight in to the chassis. If you do it this way you get the auto on off back lighting and also it will dim and brighten too!
If you need anymore help gimme a shout as I did all the wiring and feeding the boost tubing etc my self too
the end result is on my build thread
