Press R1 , R2 , R1 , Left , Right , R1 , Down, Down at the title screen (with career and action mode options) to make all unlockables available.
$10,000 and Reputation:
Press R1, R2, R1, Left, Right, R1, Up, Up at the title screen.
To unlock a given car, attain the corresponding number of reputation points.
In career mode, get the first rank.
In one career event, max out your hit meter 15 times.
In action mode, destroy your car in under 55 seconds.
In a point to point race, disable every car.
In a figure 8 jump race, win without taking any damage.
Wrangle a car in 9 seconds.
Old Tyme Racing:
In career mode, get 100 kills.
Beat every dare.
Thanks to Revolution reader D'angelo, John Kerry, and MaximusAximus!