To unlock these great bonus cars in the game you must first complete the indicated task.

Unlock Jansen Carbon X12:
Land ALL 50 Superjumps.

Unlock Montgomery Carbon Hawker:
Hit ALL 120 Billboards.

Unlock Carson Carbon GT Concept:
Find ALL 400 Smashes.

Unlock Hunter Mesquite:
Get a D license.

Unlock Nakamura SI-7:
Get a C license.

Unlock Carson Fastback:
Get a B license.

Unlock Rossolini Tempesta:
Get an A license.

If you have at least a Class A license you can enter the following passwords without the quotes as a Profile to unlock the corresponding car.

Unlock Best Buy Car:
Enter the password 'BESTBUY' as a Profile.

Unlock Circuit City Car:
Enter the password 'CIRCUITCITY' as a Profile.

Unlock Gamestop Car:
Enter the password 'GAMESTOP' as a Profile.

Unlock Wal-Mart Car:
Enter the password 'WALMART' as a Profile.