For the PC version of Split/Second enter the keys shown at the "Press Enter To Start" screen. You need to be quick and a beep indicates the code was accepted.

Corbretti Slipstream ←↓↑→→←↓↓ Corbretti Vortex GX ←↓→←↑↑↓↓ Hanzo Bayonet MX ←↓←↓→↓↓→ Ryback Coyote AMX →→←→→↓↓← Ryback Mohawk XDX ↑←→→←←→→ Hanzo Indux GSX →↑←↑↓↓←→ Ryback Brawler GSX ←↑→←↓←↓→ Hanzo FX350 CX ↑↓↑↑↓↑←↑ Corbretti Slipstream XL, Ryback Mohawk XL, Ryback Bandit XL ←←↓←←→↑↓

That said, all the cars are slower than their original counterparts, so it's not worth the effort.