To unlock Dale Earnhardt, win the Daytona 500 as Kevin Harvick (29 car).
After winning the Daytona 500, a Thunder Plate will appear with Dale Earnhardt on it.
Go to season mode and then to 'Custom Schedule'. You can unlock night races for the day tracks.
Driving locked car part:
In multi player mode, you can drive a locked car. This only works with one person at a time. Have a player choose their car, then have the other player go to the same car. Have them change to a locked car. After that, they can choose their car.