This page of our game guide to Mafia Definitive Edition explains where to find the hidden Manta Prototype vehicle in the Works Quarter 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 must explore the Central Station in the Works Quarter.
Central Station is in the central part of the district. This place is shown in the pictures above. Start by getting to the main entrance of the train station, which is depicted in the postcard.
Go to the right side of the main entrance, that is, to the north. In the spot marked in the picture Tommy can climb onto the crates and walk through a damaged section of the wall.
You'll find yourself in some sort of a maze made of crates. You need to start exploring it and head towards its northwest corner. You have to climb the crates on the way.
Ultimately, you must reach the garage door in the back shown in the picture. Kick them to break through.
Once you find the car, drive out of the garage and choose a narrow lane between the building and the small wall. You have to drive onto a ramp that will allow you to leave the fenced area.
From now on, you can choose a hidden Manta Prototype car after visiting any garage. The game does not allow you to change the color of the car body, but you can change the color of the upholstery.