The save game has to be hex editted in order to put the "Darius" vinyl and the dark side panel parts back on the car. Link to savegames below, note you need the savegame editor to "fix" the savegame for your game's cd-key:
Save with Darius Audi, standard BMW, CCX, Zonda, ... in career. Link
All the saves I make use a cd-key of 000000000000000000. You need to use the savegame editor and do a "fix" or use regedit an change the registry value for Carbon. Note the value is never checked except when you sign in online for the fist time.
I don't remember when or why I found that web site, although hex editors for savegames have been around for a long time. For example, there are savegame editors for TombRaider that in addition to increasing ammo, can place the game's character, Lara Croft, just about anywhere, to allow her to reach otherwise inaccessible areas of the game.