Guy's I know this has been asked before. But where the hell are the secret cars u unlock?

I Know the following already.......

1- I spent ages putting the secret codes in and all where accepted.
2- I know they only appear in Quick Race & MultiPlayer
3- I have checked my garage in these modes and it says it is empty
4- And the only cars in the Shop are the standard Jap, Euro & USA cars

Can anyone please explain just where exactly these cars appear in these modes?

Cheers guys and Girls