Hello so sorry nobody responded, no idea if you still play this game. But once you start the actual game there should be a car room option in the main menu, that is where all your cars will show. Its like a library. If you are looking for them in-game don't worry they load in with your save. Hope this helps :)

Cars appear randomly. So, if you go to the junkyard or a barn location that you have a map for or the auction / salon... the game will randomly choose some cars. It's just a matter of luck when you will come across the car you're wanting. Also, some cars are set to not spawn in some locations. You can edit the car config files to add in locations in the "Allowed Places" line or get the mod that does it for you....called "Allowed Places" which you can find the link to in the guides section.