Enter the below codes as the user name to get the desired effect. 

Rocket         El Nino car. 
Playtm         Toy room track. 
Xcav8          Caverns track. 
Xcntry         Auto cross track. 
Mnbeam         Space station track. 
Mcityz         Empire City. 
1Jagx          Jaguar xjr. 
Gldfsh         Underwater track. 
Seealli        Additional views. 
Playtm         Playroom track. 
Spoilt         Get all cars. 
Amgmrc         Brings out a Mercedes GTK. 

     These are to be enabled between pressing the 'start' to enter the
game  (after the opponent car selection screen) and before the loading
screen  appears.  Only  works  on  Pad1 and affects both players if in
splitscreen...

Start + Select + R1 + L2
     Makes cars around you flip around when you hit your horn.

Up + X + Triangle
     Slows down the game by 20 percent.

Select + Square + X
     Increases the weight of your car.

Up + R1 + L2
     Cops speak with different accent

Up + R2 + L1
     Cops speak with different language1

Down + R2 + L1
     Cops speak with different language2

Left + R2 + L1
     Cops speak with different language3

Right + R2 + L1 
     Cops speak with different language4

Left + Square + Circle
     25% MORE horsepower

Burnout:
     To  do an awesome burnout, put your car in neutral and put one of
the  brakes on and then floor the gas with the brake still applied and
shift into first gear with the brake and gas still on.

Gumball Policecars:
     Select  the  Redrock  Ridge track. Then before the loading screen
appears,  immediately hold Down + R1 + L2 continue to hold the buttons
until  the loading screen disappears. Now the Green Landrovers will be
replaced by Gum ball police during the race.

Power Horn:
     Press and Hold (Just until the loading screen comes up)...Start +
Select + R1 + L2

Drive a cop car:
     To  enable  this  code  pick  your  stuff, then at the RPM screen
before  the  loading screen press and hold Left, L2, R1 then hit X but
dont  hold  X  continue  to  hold  these  buttons until the RPM screen
dissappears when the game is done loading you should be a cop car. You
can't  pull  anyone  over but using the horn button will activate your
siren and lights.

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Shortcuts:

Atlantica:
     After  the  road  splits  into  two  side  by  side  at different
elevations for the second time, the shortcut is on the right, a little
before you take the regular right.

Country Woods:
     On  the  Country  Woods  track, once you get past two or three 90
degree  turns, you should come to a really sharp hairpin, (look for it
on  the  map  display) once you come to that, you see a little hill of
snow  off  to the side of the road. Go up the hill, then cut the wheel
sharp  to  realign yourself with the track. This gives you an enormous
amount of ground!

Aquatica:
     After  coming out of the second tunnel the track goes uphill, the
shortcut  is  on the uphill and to the right of the tower with the red
blinking light.

Empire City:
     The  first  shortcut  is  almost  immediately after you start the
race.  The  shortcut  is  on the left, right before you see the crane,
there is also a street light almost inside the shortcut.

     The  second shortcut is just past the first left after the crane.
You  will  see  boxes  on  the  right  with small, blinking barracades
between them. Run them over and you are in the shortcut.

     After  you  come  out  of  the  second shortcut there is a cement
barrier  directly ahead of you. Slightly to the left of this there are
several  flashing  barriers,  go through these and follow the path. Be
sure  to  turn to the left so that you can head in the right direction
on the track.

     The  fourth shortcut is after you pass through the lighted tunnel
and  a  row  of  pillars (the second long row of pillars you have come
across).  Right  after you pass all of the pillars go into the lighted
area  to the right. Stay in this area until the lights end. Right when
the  lights  end there is a miscolored part of the wall with flyers on
it. Run through the wall and you are inside the shortcut.

Hometown:
     On  the  Hometown  track,  right before the first covered bridge,
there's  a  stretch  of  grass before and in the turn. Cut through the
grass  right  before  the  turn, then let the car slide up to the left
side  of  the  track,  don't  get  too close to the side of the bridge
though.  This tip should cut off a few seconds from your time, it will
also boost your speed for the upcoming jump.

Red Rock Ridge:
     In  redrock ridge, there is a shortcut through the hairpin, right
before  it,  there is a place to pull over on the right, drive through
there,  and  powerslide left right before the cactus, then go straight
forward  through  the  cleared area, and turn left onto the road. This
helps  when  you're  being  chased  by the cops, they have to take the
hairpin, so you can lose them.