December 2017
OK SO I HAVE NEED FOR SPEED PAY BACK ON XBOX ONE X AND BEEN PLAYING FOR A FEW HOURSB NOW AND STARTED COLLECTING derelict build 1965 mustang not showing
AND AFTER COLLECETED ALLPARTS AND CHOICE IN CAR BUILD AS DRAG
POPS UP GET IN CAR SAID NO AND BAM NO CAR AND HAVE ALL PARTS TICKED OFF SO CANT FIND THEM AGAIN
December 2017
In the derelicted slot (which is shown on your first screen shot) you can also choose between different (derelicted) cars. Maybe something went wrong because you choose no, and it didn't make it to a drag racer. There is the same type of garage library like for the other 5 car slots. Try to open the car again from the library.
January 2019
I did the same. Just replace a car from the garage with the mustang. It is in the warehouse. Select a car,then select "swap with warehouse" and then you will find it unter drag