Thanks for the input, everyone!
Paul P, good advice. I ALWAYS make sure to take my wallet, phone, etc. with me and cover anything else when I have to leave my car at a trailhead (or anywhere, really). I guess my concern is having to deal with the inconvenience of dealing with a vandalized vehicle! It sounds like I should just go to the Bowen Ranch.
However, I've not taken the Bradford Ridge trail, SoCalFreeHiker. I thought I'd heard of more vandalism at that trailhead than any of the others. It would be nice to try, though.