At the 'Main' menu select the 'DNA' option and then 'Cheats and Codes' and type in the following codes without the quotes to race in a challenge which when you win will unlock the corresponding car.

Unlock Audi TT 1.8 Quattro:
Enter the code 'YTHZ'.

Unlock Holden Monaro:
Enter the code RBSG to unlock the challenge to win it.

Unlock Hyundai Coupe 2.7 V6:
Enter the code 'BSLU'.

Unlock Infinity G35:
Enter the code 'MRHC'.

Unlock Koenigsegg CCX:
Enter the code 'KDTR'.

Unlock Mitsubishi Prototype X:
Enter the code 'DOPX'.

Unlock Nissan 350Z:
Enter the code 'PRGN'.

Unlock Nissan Skyline R34 GT-R
Enter the code 'JWRS'.

Unlock Saleen S7:
Enter the code 'WIKF'.

Unlock Seat Leon Cupra R:
Enter the code 'FAMQ'.

Unlock Driver DNA Lab Car:
To get the BMW Z4 Roadster customised DNA lab car you must first enter the code. This unlocks a hidden challenge which you must then beat to get the car

Unlock BMW Z4 Roadster:
Enter the code GVDL


Unlock Last Man Standing Challenge:
Enter KNOX as a DNA Lab code.

Unlock Ascari KZ1:
Win the 'Last Man Standing' challenge to unlock the Ascari KZ1 in the lab.

Unlock Retail Challenges:
Enter JWRS as a DNA Lab code.