Unlock Jacks Knife:
At the credits screen, press X, A, X, Y, Up, L, Down, Right.
Unlock jay Jay:
At the credits screen, press L, R, A, Left, Right, Y, R, L.
Unlock Lil' Rob:
At the credits screen, press L, Left, Y, R, Right, A, Up, X.
Unlock Spitball:
At the credits screen, press Up, Up, Down, Down, L, R, L, R.