It might be an idea to contact your local Police station as you're in need of some basic crime prevention information. They have a range of leaflets and will be happy to discuss it with you. Most sensible people will not leave something of value on view and will lock cars and park in an appropriate area. What you have to consider is the local opportunists or smack heads and/or pot heads that will even break into a car if they see a carrier bag - it's worth the risk even though it might be empty. Car radios will be taken and sold for a tenner or swapped for a deal of smack or crack or pot. You need to report anything like this really and police will take an interest in the area or be able to take prints. Lots are repeat offenders, especially those of school age stealing to feed a cannabis habit. Offenders can be traced through DNA easily. I would strongly suggest you get some crime prevention advice on how not to keep making yourself an easy target. Police DO set up decoy cars, but don't know where to put them if no one can be bothered to report crime. They don't have ESP. You want something doing, you make the effort and stop expecting everyone else to do it.