I have found, that with EVERY car, especially in GT Legends, you have to start with a NEW!!!! .car file, from the downloads section. Copying a .car file won't allow it to show up, and will cause the elusive CTD.
Check the genstring in the .car file, find an existing ADD ON car, since the default cars are encrypted, find a set of wheels and windows that you like, then open its .car file with notepad, then copy its genstring. Then load up and see what happens....
Had this headache many times while painting..... Let Us know.
Regards, Jeff