At the cheatcode menu, enter one of the following codes:

Add $1 to career money total - kubmir
Unlock Argo special move - dfens
Unlock All cities in arcade mode - roadtrip or crosscountry
Unlock Bunny head - getheadl
Unlock Chrome body - haveyouseenthisboy
Unlock Faster pedestrians - urbansprawl
Unlock Flaming head - trythisathome
Unlock Increase car mass in arcade mode - hyperagro
Unlock Pumpkin head - getheadk
Unlock Roar special move - ro4r or Rjnr
Unlock Snowman head - getheadm
Unlock Skull head - getheadn
Unlock Yellow smile head - getheadj
Unlock Zone special move - allin