I guess first of all you have to rite specification for units to be designed. Automotive system is quite hard from design point of your. I have to be technological.
My understanding on automobile alarm system is when the door is open (by improper approach), the alarm system will be triggered. There is a switch at each door that will tell you whether the door is open or close. You need to find the switch and once you get it, connect the output signal wires to your alarm circuit module.
If you're interested, you can read this one, though it does not really answer your question, but it has a circuit in the automobile alarm system.