If the car happens to be in line-of-site with your windows, then maybe setup a video camera looking out at the car...
Motorbike theft is a big problem over here, and most of my friends use a battery powered baby moniter in there garage to alert them to it being broken into. If you had this you could then start the recording on the video camera. Most crime is done by a hard core few so police would probably have little problem identifying the culprits from the footage.
For a way to trace a video recorder, how about hiding a mobile phone in a void in the casing. Second hand gprs phones can be bought cheaply, and if you set it to silent then the location of the phone can be found by it gprs signal. I'd talk to your local police about this first. There are complex rules about stings to catch criminals, and they also may be able to lend you a tracking device to do this. It would be an easy way to raise their arrest rate.
Gadget