Ignition wires question

G`day chaps,

I was hooking up an amp switch for a mate the other day and was just gonna run it straight from the ACC wire on his ignition harness (was easier than dismantling his whole dash to get at the stereo wire). I was testing with a multimeter to find it, and all showed 0v exept the 12v constant. No problem. Turned the key to ACC and ALL the remaining wires showed 12v?! Turned to ON, no change. Turned off and realised all the wires showed 12v just with the key in the ignition (ie still at OFF). Why is this, and how does it work etc? Luckily I wasnt installing an alarm with immobiliser etc cos I wouldnt have a clue then, can someone explain how this is able to work?

Also, another one (this time I WAS installing an alarm). I was connecting the immboliser (just a relay across the START wire.  Found the right wire, cut it, yet the car still starts (albeit taking a bit longer to start). Why is THIS? Its definately the right wire, so I cant figure that one out either.

Every other car Ive worked on (although only half a doz or so) have had normal ignitions with a definate 12v, ACC, ON, and START wire, which is why these 2 have me stumped.

Any help would be great!

Cheers

- Dan