This page of our game guide to Mafia Definitive Edition explains where to find the hidden Lassiter V16 Appolyon vehicle in the Beech Hill 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 luxury properties in the Beech Hill district.
The property in the postcard is the one at the eastern end of the map - the accompanying screenshots show it, but you won't find the hidden car in that exact spot.
A hint from the postcard tells you to visit the neighbours. So step down from the steep driveway and take an interest in the garage on the opposite side of the road, which is the one in front of the property with the blue facade. This is where the hidden car is stored.
Inside the garage there is a person that will attack Tommy. Be ready to shoot the enemy or eliminate him in melee combat.
You can choose the Lassiter V16 Appolyon car after visiting any garage. The game allows you to change the color of the body and the color of the car's upholstery.