hire a garage for the car? or some where secure for it, stick a camera on it to cctv. Its probably someone you know, maybe a ex.
You only called the police about it 3 times, right? The police are like any other bunch of public servants: they will pay the most attention to whoever whinges the most. Call them about it again, ask them what they are doing to follow up the case. Call them every week for an update until they do something.
Ahahahahahahaaaa!! There really is a lot ot be said for your approach, Pete. There is a certain elegance about this solution.