The toad AI606 comes with an armed output that puls to ground when you arm the alarm that u can use for an LED get 1 from Maplins and ask em for a suitable resistor so it doesn't burn out, they do them in just about every colour, and if i were u i'd go for a wide angle LED.
All you do is put 12v feed to one side of the led and conect the other side to the alarm.