The Simpsons: Hit & Run Cheats

Fail a mission 5 times and pick the skip a mission option it the menu.

Level 1 - Rocket Car - Found in front of the solid gold house. (From episode "The Day the Violence Died")

Level 2 - Monorail - Found at the rubble of the monorail station, the front car. (From episode "Marge vs. the Monorail")

Level 3 - Knight Boat - Found aboard the C-Spanker, in a cargo crate. (From episode "And Maggie Makes Three")

Level 4 - Four-wheel motorbike - At Royal King trailer park (episode unknown)

Level 5 - Obliteratatron - Outside Springfield Stadium. (episode unknown)

Level 6 - Pink Cadillac - crashed into wal of Planet Hype (From episode "Homer's Barbershop Quartet")

Level 7 - Remote Control Car - On roof of Krusty Burger (episode unknown)