Here's an idea!!
I usually go out of my way to make my car unappealling to a potential thief. (Probably not as gross as dr_wu002's, however, but the same concept!)
I toss a pair of dirty socks and underware briefs up on the dash. I drape a foul T-shirt over the steering wheel as if drying. I leave lots of trash on the seats, and I also leave the glove box open. Anything of real value (CDs, etc) goes under the front seats.
I hike with my wallet, credit cards, and keys in my pack (oh, ...the extra weight!) and hope karma works.
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Last winter I was sleeping in the car at the Hancock trailhead on the Kanc. About 2 AM a junky Toyota with a loud exhaust pulled in and drove up next to my car. I head an occupant say, "someone's sleeping in it."
They moved down to the very end of the lot next to where another (empty) car was parked. They shut off the engine and stayed there for about 20 min, then drove off. In the morning I noticed a few beer cans and roaches in the snow where they had parked.
I assume had I not been sleeping there they would have whacked that other car and been on there way.
Maybe if the state let folks sleep at trailheads there would be fewer breakins!!
cb