I didn't read all the pages on here, but here's an idea... If they do leave their car doors unloceked at night -- take it and put it in the dash vents... Just break it up some, and sprinkle it in the vents on the dash...
You would be surprised what it smells like after the car warms up in the winter !!!!
Sometimes I lie awake at night, and I ask, 'Where have I gone wrong?' Then a voice says to me, 'This is going to take more than one night.'