First challenge razor and beat all his races (which may take some time) then complete the persuit afterwards. Skip all the credits then customize a car, just pick anyone to try it out. You will now have 2 new spoilers, 2 new rims, 1 new hood, 13 new vinyls (can be found in unique), a new awesome gauge and a massive 8 mile circuit map.

Also if you really want to get more parts beat every other racer and choose the marker of the eye (vinyls etc.) and the other sign (which should show something like a car with new bumbers or something) and you'll get the unavailable parts you can't get in the game, these include the blacklist rivals Decals, Vinyls and if your lucky you'll get their pinckslip for their car. You can then bring that car to the garage and change the speed and look.