The pins go from 1 to 17 from left to right as you're looking from the inside of the phatbox towards the back.
Pin 13 is ground.
Pin 15 (Regulator 2) should be 5V and always on when the "ignition' is on. This powers the microcontroller.
Pin 4 (Regulator 4) is 3.3V, and only on when the PB is powered-up by the head unit. This is the supply for the ARM, SDRAM, Flash, etc.
Pin 2 (Regulator 3) is 5V, and is also only on when the PB is powered-up by the head unit.
Pin 1 (Regulator 1) is 8.5V, and I'm not sure it gets used for anything (it might not even be enabled, I haven't checked).