Successfully complete championship mode without using any tokens to unlock all tracks, cars, skins, and other cheat options.
In 2-player ass battle, pick your cars and go to a stage(it works with every one). Wait for the mayor to keep on talking and you will hear Player1 say something!
Note: each car will say something different.
How to get other cars
Rally Days 1 - Win the race without getting a pick up (Cheat Sheet).
Rally Days 2 - Activate Checkpoints 1 and 4 only (Pip). Activate all four checkpoints (Mr. Garrison).
Cowdays - Lose the race without ever touching the cure (Bebe).
Valentine's Day - Collect the three hidden pick ups (Shelly). There is one on each tower and one behind the Skater picture in the jungle.
Spring Cleaning - Collect 5 caffeine pick-ups (Tweek). Win the race (Mr. Mackey).
Read A Book Day - Hit Chicken Lover 5 times with the salty balls (Cartman Cop).
Easter - Nothing to unlock.
Pink Lemonade - Win the race (Big Gay Al).
Memorial Day - Get the hidden pick up on top of the plane (Ike). Collect the 2 hidden pick-ups (1 is above checkpoint #1, and the other is between checkpoint #4 and the wooden bridge) to unlock Visitor.
Independence Day - Win the race (Mephesto). Collect 13 or more turbo pick ups (caffeine, terrance, Philip etc) (Ned).
Halloween - Win the race (Grandpa). Win the race while only dropping off four candies at a time (Death).
Thanksgiving - Lose the race with out getting a single turkey (Marvin).
Christmas - Collect the 4 hidden pick ups (Terrance and Phillip). Win the race (Jesus).
New Years - Win the race (Satan). Win the race being the only one to touch the millennium key (Damien).