get a long piece of wire, , go to a hobby shop and ask for rocket engine ignitors ( they are cheap like $2 ) get a multipurpose screwdriver..
Attach the ignitors to the wire , unscrew the tail brake-light of this guys car smash the bulb and attach the wires to the piece of wire you have with the ignitor at the opposite end. Open his gas tank and insert the ignitor into the tank , replace the taillight cover and tape the wire to the car body so it can't be seen.
When the person who owns the car hits his brakes * KABOOM
If you want to be polite , simply attach a firecracker on the ignitor and cut the wire leading to the gas tank , and leave a message ( "next time the wires will be connected and no firecracker
")
DISCLAIMER. this post is for educational purposes only, any attempt to make such a device could cause fatal injury .