This page of our game guide to Mafia Definitive Edition explains where to find the hidden Trautenberg Sport vehicle in the Downtown 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 Waterfront location at the western end of Downtown district.
Before you reach for the hidden car, take any firearms with you (visit Vincenzo's Armory). It will be required to unlock the access to the vehicle.
Once you reach the Waterfront walk through the wide promenade running along the water. The destination is an adjacent pedestrian area with three gates shown in the pictures above.
Walk to the right gate and notice that Tommy can break down the door.
You'll find yourself in a garage with a lot of crates. Start using firearms to destroy the crates and unlock further passage. Don't worry that the gunfire may alert the police, because you cannot be found by law enforcement officers here.
Go a bit further and shoot another crates on the top. This will allow you to climb onto the lower crates.
You'll reach the secret car, but you still have to unlock the garage door. There's a simple puzzle to solve here with the switches as shown in the picture. Press them in the order indicated by the postcard - middle switch, left switch, right switch.
By restoring power to the gate, you can now get in the car and drive it out of the warehouse.
You can choose the Trautenberg Sport car after visiting any garage. The game allows you to change the color of the body and the color of the car's upholstery.