If I were in an area where vehicles were stolen often, I'd totally booby trap my car.
Maybe a couple of razor sharp Gaff hooks mounted on a swiveling bracket under the seat.
You'd find them in the morning with a pair of gaffs in their ass, crying, bleeding... Unable to move, and tired from holding their weight off of the hooks, having been fighting it all night.
Sure its against the law, but would a jury convict on it?