To unlock other cars, you need to find three keys per level. When you begin
a level, it is a good idea to find all the keys, then complete the laps. It
does not matter what you place when you are looking for keys; take your time,
and complete the level. If you do not complete the level and found all the
keys, do not worry -- you still get the prize. However to go onto the next
level,you must place third or higher.
In the third Grand Prix, in the second to last level "Tomb And Gloom" is where
you can unlock the find fastest car once you have found all the keys. The third
and last key is hidden well here. It is in a curtain draped off the ground in a
descending matter (just hop of the ledge onto the curtain). A good rule of thumb
when looking for keys is to change the cars view to a more elevated position,
then try to spot the sparkling keys in the distance.