Agree with the guys that smushing fish into the vents is great revenge…but if think that a “red herring” should be placed into the car so that the person will find and think that lingering odor is from the found fish…
That’s pretty devious. I hope I never get on your bad side 
Once, I had a bunch of broccoli tumble out of my grocery bag and into the trunk. In less than a week there was a nasty odor in the car. While the vents would be the #1 choice, your red herring under the trunk carpet would be my pick for #2 spot.
Doing this to a car can do $100s of dollars of damage, and the smell might never come out. Way too extreme.
I work for the Tribes in Oregon as a fisheries biologist, and we are constantly “sliming” our cars from fish carcasses, wet waders with fish slime etc. I learned from a tribal member that if cut fresh pine-boughs, and leave them in your car (right on the seat in this case), the smell will be gone overnight. It works better than anything you can buy at the store. Tested and True.
This is one guy you have to have back on if he does get revenge on his buddies
Every time you discuss car odors, I am surprised you never mention this solution, suggested by my mother. My husband once worked for a company that sold fish, and it happened that some fish juices seeped into the lining of the trunk of his car. We tried everything and thought we would have to abandon the car on the side of the road somewhere, but my mother’s suggestion worked: charcoal briquets. Just put them in an open container, deep enough that they won’t spill out, and leave them there until the odor goes away. This really works!
The other method I heard of (similar to the briquets) was to fill a small bucket with a Mix of Ammonia and water.
I never had the need to ever try this solution…and I hope I never will.
Strap the fish to the underside of the accelerator. Every time you hit the gas, you squish the fish and release more odor, but the pedal hides the evidence.
I’m gonna regret this, my conscience will bother me. This will work on cloth seat covers only. Get a small fish and wait till its good and slimy. Now, pull up the seat cover on the drivers side, put down some plastic, the fish on top and then the seat cover.
The plastic will keep the fish juices from going down into the seat padding, but when the driver sits down, the juices, which can’t go down, will go up.
Now the driver will have a rather unflattering stain on their pants as well as an unpleasant odor. It will look like one of those embarrassing accidents.
Fish oil capsules anywhere that gets hot work quite well. They fit wonderfully in overhead lighting fixtures if you felt compelled to bring the battle inside. It’s hard to track down but it’s a bit easier to clean up. There are also plenty of crazy-making devices for pretty cheap out there. A favorite is: http://www.thinkgeek.com/gadgets/electronic/b278/
Happy pranking (and prepare yourself for further madness)!
Ray recommended isopropyl alcohol for to get rid of the smell based on getting rid of skunk smell. I’ve heard of another cure using Hydrogen Peroxide. Here’s a link if anyone’s interested:
http://dan.drydog.com/patsyann/skunk.html
We have allot of skunks where I live and I’ve been very lucky not to need it but I got it just in case. Might it work for fish? Don’t know, but it might be worth a try.
Here’a another question that no one seems to ask:
What would you do to someone who put a dead fish in your car? Remember we’re already discussing and in many cases endorsing* committing a crime in this thread, so the sky’s the limit.
*moderators seem to be fine with that, go figure.
No, actually, it’s something that’s gotten our attention before. We don’t endorse any of this…including the discussions about deterring gas thieves, throwing back lit cigarette butts into a car, modifying license plates, and driving on other people’s property. You do have a point, piter. We’ve not aggressively removed these posts, but it’s something we need to look at as we’re updating the community guidelines.
Limburger cheese on the manifold - no harm - just foul.