To unlock all create-a-car sponsors, win the Winston Cup in Career Mode and you'll get a thunder plate that states all create-a-car sponsors are unlocked.

To unlock Kitt Paint, win the Winston Cup in Career Mode.

To unlock paint skins, set the game level on Veteran or Legend and then race on any 1 player race. This will not work on 2 player races.

Win the Daytona 500 in season mode driving Kevin Harvick's #29 car to unlock Dale Earnhardt. You will also get the Goodwrench car, Goodwrench Plus car, Wrangler car, Silver car, Coke car, Bass Pro Shops car, Oreo car, and the Legacy car.

Reach Level 16 to unlock the All-Star Race. Reach Level 18 to unlock the Bud Shootout.
Select the Features option on the main menu, then Create-a-Car. Enter the name Seymore Cameos as printed.