After reading this thread i decided to have a look at the centeral locking system in my magna, what i found was that when the doors unlock one of the wires comming from the controll box would go high for about a second,
my Carputer uses a fairly basic shutdown circuit and boots once power is applied to the Acc line. once the acc line goes off the computer powers off after about 15 minutes,
all that i had to do was put a wire comming from the central locking wire to my ACC line of my Carputer, with 2 diodes in in it the first diode was between the centeral locking and the acc line to stop power flowing back into the centeral locking, and the second one was inline in the acc line to stop power flowing back into the ignition line of the car.
it is very simple and works great!