To get the F1 car, complete a circuit in 1st place.

Now enter One race or circuit (I recommend one race) and on the track selection screen, press Z 4 times. You should hear a sound. Now select a track to go to the Car Selection screen. The hotrod (if you got it) is replaced by the F1. If you want to switch back to Hotrod, re-enter the code (Z,Z,Z,Z) on the car select screen.