This page of our game guide to Mafia Definitive Edition explains where to find the hidden Flame Spear vehicle in the Central Island district of the City of Lost Heaven. It is one of the five hidden cars in the game.
From the postcard associated with this hidden car you can find out that you have to get to the construction site in the Central Island district.
The construction site is in the southern part of the Central Island district. You have to find a skyscraper that is under construction - check the attached screenshots.
Go around the construction site until you find a garage with a secret vehicle shown in the picture.
From this point on, you can choose Flame Spear hidden car after visiting any garage. Unfortunately, the game does not allow you to modify its appearance.