Well your door remote tells your alarm brain to send a signal out a certain port and down a wire connected to that port. What you would need to do is connect that wire to the on/off pins on your motherboard so they get jumped whenever you hit the button. Furthermore, you'd have to somehow regulate the voltage in that wire to be the proper amount needed to jump the pins.
This is simply my assumption of how it should work.
Upon doing a little searching, why would you cross post?
And why would you need this if you're selling your car PC?