See if your mechanic can install a little red flashing light. It make the car looked armed with an alarm.

When I lived in mississauga several cars all around mine were broken into (windows smashed, things stolen). My car was the ONLY car that had a fake alarm (the flashing light) & it was one of the very few not broken into.

Everyone that goes in my car sees it & when we leave they remind me to set the alarm.. HA! As soon as the car is off the light flashes non-stop. Once the car is on it auto turns off. Its auto built into my car, so I don't know how easy it would be to install just the light.

Trash box or not, it still sucks having your car broken into