When you have successfully collected ALL the playing cards in Story mode the hidden Bonus game will become unlocked.

Purchase the following costumes at their corresponding levels and locations.

Homer Casual - Level 1:
100 Coins (Purchase at the Kwik-E-Mart or the School)

Homer Chosen One - Level 1:
150 Coins (Purchase at the Kwik-E-Mart or the School)

Homer MuuMuu - Level 1:
200 Coins (Purchase at the Kwik-E-Mart or the School)

Bart Tall - Level 2:
150 Coins (Purchase at Moe's Tavern or the DMV)

Bart Fotball - Level 2:
200 Coins (Purchase at Moe's Tavern or the DMV)

Bart Ninja - Level 2:
250 Coins (Purchase at Moe's Tavern or the DMV)

Lisa Cool - Level 3:
250 Coins (Purchase at Android's Dungeon or the Observatory)

Lisa Floreda - Level 3:
250 Coins (Purchase at Android's Dungeon or the Observatory)

Lisa Hockey - Level 3:
300 Coins (Purchase at Android's Dungeon or the Observatory)

Marge Inmate - Level 4:
300 Coins (Purchase at the School or the Kwik-E-Mart)

Marge Classy - Level 4:
350 Coins (Purchase at the School or the Kwik-E-Mart)

Marge Police - Level 4:
400 Coins (Purchase at the School or the Kwik-E-Mart)

Apu Army - Level 5:
375 Coins (Purchase at Moe's Tavern or the DMV)

Apu American - Level 5:
425 Coins (Purchase at Moe's Tavern or the DMV)

Apu B-Sharps - Level 5:
475 Coins (Purchase at Moe's Tavern or the DMV)

Bart Hugo - Level 6:
400 Coins (Purchase at Android's Dungeon or the Observatory

Bart Cadet - Level 6:
500 Coins (Purchase at Android's Dungeon or the Observatory)

Bart Bartman - Level 6:
600 Coins (Purchase at Android's Dungeon or the Observatory)

Homer Dirty - Level 7:
400 Coins (Purchased at the School or the Kwik-E-Mart)

Homer Evil - Level 7:
450 Coins (Purchased at the School or the Kwik-E-Mart)

Homer Donut - Level 7:
500 Coins (Purchased at the School or the Kwik-E-Mart)

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Cheat Codes
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For all of these codes, you need to hold L1 and R1 and enter them at the options menu in living room:

Fastest Cars:
Triangle,Triangle,Triangle,Triangle.

So for the fastest cars (above) cheat to work, you need to go to the options menu, hold down L1 and R1 and then press the triangle button four times! Use a similar method for the rest of the cheat codes.

Triangle,Triangle,Square,Square - Blow up vehicles in one hit

Square,Square,Square,Square - Faster Cars

Circle,Circle,Circle,X - More camera angles

Triangle,X,Triangle,X - Infinate car health

Square,Square,Square,Triangle - Press horn to do a giant leap in your car

Circle,X,Circle,Triangle - Grid Mode

X,Circle,X,Circle - All reward cars

X,Square,Square,Triangle - Credits

Triangle,Triangle,Circle,Square - Speed meter Display

Circle,Circle,Triangle,Square - Red Brick Car

X,X,X,X - Night Mode

Triangle,Circle,Triangle,Circle - Tripping Mode

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Unlock Cars
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Level 1(homer):rocket car (in gold mansion driveway)
Level 2(Bart):Monorail (up stairs downtown)
Level 3(lisa):boat (on big boat)
Level 4 (marge):Quad (in king trailer park)
Level 5 (Apu):Monster truck (near foantain it's on display)

At the main menu, go to the options screen.

Hold down L1+R1, then press X, CIRCLE, X, CIRCLE to activate the cheat. A horn will confirm entry

At the options screen hold L1+R1 and enter:
Triangle, X, Triangle, X - For Infinite car health

At the options screen hold L1+R1 and enter:
Triangle, Triangle, Triangle, Triangle - For Fast cars

At the options screen hold L1+R1 and enter:
Triangle, Triangle, Square, Square - Blow up vehicles