I will try to explain:
First, choose any car you want from the Secret Menu (i choose the NSX Type R '02 as a example). Save the game and quit it.
Second, decrypt your save with Bruteforce and then open up your save and after you are in HxD use the search option to found 01 8C 0F.
After you found it we will try to replace with a standard car (I choose the Viper SRT10 '03) which the value is 02 3A 0F.
After this encrypt your save and you are done it!
It's been a while since i do this, so the tuto posted above could not work.