Here is a list of some cool ideas for car related pranks. You do these pranks at your own risk though, so don't come yelling at me if it doesn't go how you plan it.

Step 1: Exhaust Whistle

An exhaust whistle is very simple, you put it in the exhaust pipe of your friends car and when they start the car and accelerate it starts making a noise which makes them think that they may have a mechanical issue with their car. Creates some funny reactions!

Available for $2.67 here.
http://www.amazon.com/Morris-Trick-Exhaust-Whistle/dp/B0006GK1XY

Step 2: Car Replacement

If you have a decent amount of time and a friend or family member who is obsessed with their car, you can move their car one street over, go to the junkyard and get a body of the same model and color and switch their car. They will come home and their car will seem to have been demolished.

Step 3: License Plate Frame Change

This one is simple, fast, and harmless just like the exhaust whistle. You switch out a friends license plate for something else that is funny. For example, I did this one to a friend who is a guy and is a chargers fan. I found a pink raiders license plate at a yard sale for $1 and put the girly license plate on his new tough and manly dodge truck. Let's see how long it takes him to notice :D.

Step 4: Honk If You Like My Braking

This one is fairly simple. All you do is get an old horn for a car that still works and wire it up to the same wires as the victim's car's brake lights so that when they put on their brakes, it honks the horn. You could also do the same thing so that when they start the car it honks by wiring a horn to the same wires as the car's electric starter motor.

Step 5: Switch Up the Settings

Turn on the radio full blast so it is loud when they turn it on or change all of their preset stations to opera or political talk-shows. Get creative, turn on blinkers, windshield wipers, adjust the seat so they are sitting with their face in the windshield, etc.

Step 6: Rock N' Roll

This one is similar to the exhaust brake in that it is simple and harmless and makes people believe that their car is broken. Rather than making a screeching noise, it makes a clunking noise. All you do is take some rocks and put them in the hub caps so that when the person starts driving, the rocks roll around and make noise.

Step 7: Hidden Noisemakers

There is a device called the Annoy-a-tron that makes a high pitched noise at short and random intervals making it hard to detect. You can hide one of these somewhere clever in the victim's car. This also works well with musical greeting cards. The more annoying the song the better.

Annoy-a-tron tutorials
https://www.instructables.com/id/Homemade-Annoy-a-thing-Annoy-a-tron/

Made with Arduino:
https://www.instructables.com/id/Arduino-Annoy-a-tron/

Step 8: Fill It Up

You can fill their car up with all kinds of interesting things. Cars can be filled up with balloons, confetti, and almost anything you can think of. If the person has a truck parked in a store parking lot, it is fun to load a shopping cart into the truck bed and they will be quite confused when they come out.

Step 9: Unclip the Battery

Pretty self explanatory, just pop the hood and undo one of the battery clips and the car won't even try to start.

Step 10: Fake Traffic Ticket

You can make a fake fine to send someone through the mail (link below) or you can buy fake parking tickets to go directly on the windshield on many various websites.

Template for traffic ticket through the mail:
https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B3vYm2SdCUn3NWMxMzhhMzgtNmVjNS00ZDY2LTgzNDEtZTlkMTM3NjI2NWM0&hl=en

Step 11: Now That's Just Jacked Up!

You can put a car on jack stands just enough so that the driving wheels are like 1/4" off the ground. If the jack stands are painted a dark grey, they may not even be noticed. When the victim gets in the car, they will throttle it and it will go nowhere. FOR SAFETY USE JACK STANDS NOT HYDRAULIC FLOOR JACKS AS HYDRAULIC FLOOR JACKS ARE NOT STABLE

Step 12: For Sale

Self explanatory, put a for sale sign on the victims car or post a for sale ad on Craigslist.Make it the price as cheap as you can while still being believable. The victim will start getting tons of calls flooding in! If you want you can put late night calls acceptable on the Craigslist ad also.