Some people really love their cars so you will want to be careful when selecting a victim. Some people name their cars, protect their cars and use their cars as part of their identity. These are the funniest people to screw with. With victims like this you will want to be anonymous.
One friendly silly way is put post it notes on every inch of it. Another one of the classic ways to do this is to paste a rude bumper sticker on their car. You should find one that says "honk if your gay" or "I hate cops". Use your imagination and have some fun. It also cheeses people off if you put their opposite party on their car. If you want funny political bumper stickers, these are the best around. Click here to go to Liberty Stickers
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EVIL PRANK BONUS
It is also fun to trick your victim into thinking something is really wrong with his car. One of the funniest things to do is to use the old classic Exhaust Whistle Gag. You put this small device in the exhaust pipe and it whistles like mad. You can find it here. Exhaust Whistle GagAnother way to do this is to tie some empty cans underneath the under carriage. You will want to tie them deep under the car so the victim can't see them. This will make a hell of a racket on a bumpy road and the victim will think something major is wrong with their car.
GETTING EVEN BONUS
Plant Fun snaps under all the tires. If you don't know what these are you can read about them here. See Firecracker Revenge
When these things go off the victim will think something is wrong with their car. Combine the fun snaps with putting a condom around the exhaust pipe. The condom will inflate and blow up.
GETTING SERIOUS REVENGE BONUS
Steal your housemate's keys and pop the hood of their car. You can buy a special firecracker made to fake a car fire. It will smoke like crazy and mimic a fire under the hood. If you want to be a complete jerk stuff a bunch of bananas in the tail pipe too. This causes the oxygen sensor to think there is something wrong and the car will run badly. If you are lucky it will stall out while the smoke bomb is going off under the hood.