At the cheat menu enter YAYCARS.

All Unverified.

The cheat menu supposedly accepts all the following codes:

MATTL66 ... Unlock all Art
FRANKSB ... Play as Tractor
PEACEOT ... Play as Fillmore
CASEDEL ... Play as Luigi
BOOTCMP ... Play as Sarge
CHEATER ... Play as Boost
MUSIC8R ... Play as Dj
OLDY2D0 ... Play as Lizzie
MONAR02 ... Play as Monster truck #1
GOOGLER ... Play as Monster truck #2
RACEKAR ... Play as Flecher
PIXARFY ... Play as Woody the Wagon
MCQNL94 ... Play as Mia and Tia
DUN33LI ... Play as the #33 racecar
TORCHED ... Play on Fire