To gain access to the Extra 1 option on the main menu, you must first complete the Beginner's, Medium, and High Speed Classes in Championship mode. I'm not sure exactly what the requirements, cup-wise (gold, silver, bronze), are though; I had all Golds on all tracks when I opened up this options.

Hold the L and R buttons, then press Down/Right + B.

At the password screen: Complete each of the races with a gold medal

Enter the following codes at the startup screen (where it says Press Start):

- To unlock the Indy Car Mode, Press and Hold L+R, then press Left on the D-
Pad, then press B. When you hear a sound press start and it will be unlocked.

- To unlock the Go Kart Mode, Press and Hold L+R, then press Right on the
D-Pad, then press B. When you hear a sound press start and it will be
unlocked.

- For all tuning parts, press and hold L+R then Down+Right (diagonal) then
press B.

At the password screen, enter the appropriate password (case-sensitive):