Ultrasonic sensors seem to be the sensors of choice for many car alarms. You can often buy electronics kits which use ultrasonic sensors to detect motion. The outputs are often simply in the form of a relay, so you can connect it to almost any other device you can think of. Google around for 'ultrasonic motion detector electronics kit'. There's a bunch to choose from.
Simply mount one of these inside the car, and wire it up to the 'deterrent' transducer on the outside.
Alternatively, you could simply wire it up to a wireless doorbell, and leave the chimer by your bed at night.