It does happen quite a bit here in the Pacific Northwest. Some trailheads are more vulnerable than others. For your location, check with the www.nwhikers.net forum. So far, I've been lucky although on a couple of occasions the car parked next to mine was broken into.

Rule #1: Leave nothing, of value or otherwise, in your car. This includes any clue to your identity such as the vehicle registration card. Better to carry that with you. An obviously empty car is less apt to be broken into. Please don't reward the car clouters!

It also appears to help to leave the glove compartment and any other compartments (like the one between the front seats) open to demonstrate that there's nothing inside.

Some suggest leaving the vehicle unlocked. Unfortunately, some clouters automatically smash the windows without even trying the door. It might be a bit hard to explain theft from an unlocked car to your insurance company.

Supposedly the most foolproof way to avoid theft is to drive a 20-year-old beat up car with nothing in it and leave it unlocked with windows partly open. I can't do this since such a car requires a lot of mechanical tinkering to keep it going. My car repair skills are limited to raising the hood, looking at the motor, and putting the hood back down.