look for a DEI 629L they are pretty much sold out everywhere but a couple places might have them
it is a bright blue 5 led flasher that you connect to pretty much any alarm, all it needs is a ground and the red wire goes to the alarm brain.
the led is activated when the alarm is armed. then you just place the led in a obvious viewable place, i put mine in the center console on the yukon and near the ignition on my pickup
anyone that looks at the vehicle will know it has a alrm and are likely to just pass it up and move on to easier prey
this is what it looks like in my silverado