Bugger, should learn to proof read my posts, here's the correct version:
Rob to make life easy for yourself rather than guessing at resistor values and wiring just buy a 12v flashing LED and wire it permanently into a 12V supply anywhere in the car, the 20mA it uses each pulse is a lot less than any alarm system will likely use so just leave it permanently connected.
If you do want it switch it with the ignition then connect the + side of the LED to ground, earth or -12v (whatever you want to call it) then connect the NEGATIVE terminal of the LED to the switched +12v supply that feeds your stereo, that's easily accessible in most cars and just requires you to route the cable for the LED behind the dashboard somwhere use a scotchblock to splice into this wire if you can't or don't know how to solder.
Chris