From Mel in Mitchell, South Dakota:
When I was in college, my college roommate, who I love like a brother, had a Jeep that he worshipped. I would make Cherry Kool Aid and pour it under the Jeep to make him think he had a transmission leak. I would strew miscellaneous nuts and bolts of appropriate size to make him think something was coming loose underneath the Jeep. While I don't remember the exact part, I once went to a junkyard and taped a spare piece under the Jeep. When it fell off it made a noise, and naturally he stopped, debating whether or not it was safe to drive. All of these are safe, nonviolent ways to get back at the jerk who left the fish in Mark's vents - without property damage. The emotional damage and the worry, to quote Mastercard's ads, is priceless.
From Doug Williams:
During a period of practical joke wars with a friend, I discovered a great way to foul a car in a way that's hard to trace.
I work in the health food industry and there is an herb called Valerian root, that is commonly sold for relaxing muscles. It has the distinct smell of stinky feet. I took a few bags of Valerian root tea and sprinkled it over the air intake vents of the car. Among leaves and dirt on the car, this does not draw attention. Dry, the smell is certainly noticeable. The great thing is that when they wash the car, and the herb gets wet, the resulting herb tea becomes even more intense. The smell is foul enough not to be missed, but non-distinct enough, and close enough to body odor, that it is not likely to make the owner realize they have been pranked. The smell eventually goes away... after 6-12 months.