Well, I’ve had my car broken into – a few weeks after I bought it new, before I’d had time to get an alarm fitted. Several other cars in the car parking area (at the start of a popular bush walking track, close to a city suburb, but well away from houses) were also broken into at the same time – the only one not attacked had an alarm – it obviously deterred them from damaging that car. Honda didn’t have any of the large front door (3 door Civic) windows in Australia, so I had to wait more than a week until one arrived from NZ. Then I got an alarm fitted, and despite being parked regularly in the street at night, I’ve never had it broken into again in the past 12 years. Some other cars in the street have been damaged overnight, although it happens fairly infrequently (every 10 years or so!), but they now stay away from mine.

So I’d get the alarm first, before the central locking. If you can’t afford an alarm, an effective deterrent would be a flashing LED, as quangsuke said, and a couple of alarm warning stickers, which would make most thieves think twice about damaging your car, for less than $10. They are available from Jaycar - see here http://www.jaycar.com.au/productView...T&SUBCATID=288 and here http://www.jaycar.com.au/productView...T&SUBCATID=693 . It's not just car thieves they deter - people after a bit of money will often break in, if they think you might have left something in the car to steal, even if you haven't, and the damage they cause costs money to fix - it's often not worth claiming on insurance. A chap I worked with who'd had his old car broken into many times while he was at work used to deliberately leave his glove box open, to show the thieves who targetted that area, that there wasn't anything in his car worth stealing, to save himself broken windows!