A couple of xmas ago, some friends stayed at our place and got their car broken into during the night in our driveway, so I decided we needed a car alarm. As an amateur, I went for the homebuilt stuff.
I got the cheapest remote door bell I could find ($15 at Bunnings!). Since the remote works on a 12v battery, I stuck the button pressed in on the remote and connected the battery poles to the indoor carlight, so that when you open the door of the car, the door bell rings. It also serves as a bell boy call so that when the XYL gets home, I can come down pick up the groceries. It actually works if the car is parked few houses away.
Since I got the cheapest one, it can’t be user programmed (unless you use a soldering iron, etc) and most of the ones at Bunnings had the same code. This means that everytime I get into the car, I’m probably ringing at some houses on the street, and if I drive with a door not closed properly (typically the tailgate) then I’m probably ringing at many doors on my way.
Still don’t know what I’ll do if it rings during the night, the car is parked just in front of the first floor bedroom balcony, perhaps I should keep a bucket of water on the balcony!