how can I get the code number for my cars theftlock audo system. I bought my car second hand

I'm having a problem with the theft system light coming on and flashing on my dash and as long as it stays flashing my car will start up and die, start up and die, then finaly the theft system light will stop flashing and then my car will start and run fine, but it's starting to mess with me more and more, I never know when it come on and prevent me from driving my car. I bought a after market manual and all it says is that if my car has a theftlock audio system, that I need to know the code number for it, Dose any one know how I can get the code number for the theft system in my car??