Pulsed output

is the option output a low or a high pulse?

This *should* work in theory. After recieving a signal from the option input, it will pulse the output high for .1 sec, low for .4 seconds and keep doing so for about 30 seconds. The timing intervals can be changed by changing R4 and R5, if you would like a variable rate i would suggest a potentiometer (like a 0-10K) in series with a 100K resistor for R4. Also the value of R1 will determin how long the output keeps pulsing (the higher R1 is the longer). The output i would assume you want to hook to a relay. Remember this will only give you pulsing DC, so if the siren requires an AC driver it wont really work. Also i take no responsibility for this thing if it destroys the alarm and/or car, use this at your will. If you want i can write a description of how the circuit works.

Also, this is not the best or most reliable circuit, but it is the most simple way of doing it i could think of.

wait i messed up the transistor at the end, in that circuit when the circuit is off, the siren will always sound, not good, here is a better setup, notice it is a negative output though. no problem really, i also put a relay in to show how it would be hooked up. Note that is and NPN transistor at the far left and a PNP in the middle.