El Cajon Track:

At the Track Selection screen, hold Up and press L
and R at the same time. You should hear an air 
wrench if done correctly. Select El Paso, hold Z 
and press the accelerator button. 

Flagstaff Track:

At the Track Selection screen, hold Left and press
L. You should hear an air wrench if done correctly.
Select Mojave, hold Z and press the accelerator 
button. 

Guadalupe Track:

At the Track Selection screen, hold Down and press
R. You should hear an air wrench if done correctly.
Select Vegas, hold Z and press the accelerator 
button. 

Monster Trucks:

At the Car Select screen, press the respective 
buttons to switch between the default and monster
truck cars. 

Car - Button

Punisher - C-DOWN 
The Crusher - C-RIGHT 
Thunderbolt - C-LEFT 
4x4 Monster - C-UP