Muscle Car: 
Find 4 keys in any track; press down at gear 
selection screen. 

Hot Rod (Rocket Car): 
Find 8 more keys in the other tracks; press 
down at the gear selection screen. 

City Bus: 
Find all 16 keys; press down at gear selection 
screen. 

Police Car: 
Finish 1 player GP Mode; press down at gear
selection screen. 

Taxi Cab: 
Finish 1 player Circut mode; press down at gear
selection screen. 

by Ben Richard