In  the  scene  where  you  choose  to  go to map select, vehicle
select,  start  race etc, type ENABLE to be given access to all tracks
and vehicles. You can now use all of the other cheats in the game

     When the race starts, press F4 and find cheat mode. Now use these
cheats:

F5                      Total repair (free)
F6                      Toggle Invulnerability on/off
F7                      Adds 30 seconds to the timer
F8                      Freeze/Unfreeze timer
F10                     Increment lap counter
F11                     Earn 5000 credits instantly
SHIFT-F6                Show opponents on map
SHIFT-F7                Adds 300 seconds to timer

Also try these:
# (any number)
SHIFT-#
ALT-#
SHIFT-ALT-#
CTRL-#