This page of our game guide to Mafia Definitive Edition explains where to find a hidden Disorder vehicle in the northern county. It is one of the five hidden cars in the game.
From the postcard associated with the hidden car, you'll learn that you have to find the Vevoda Motel in the North County, which is a rural area north of the City of Lost Heaven.
The motel is located at the northern end of the map, east of the Fyer Dam. The exact location is shown in the screenshots above.
It's very easy to find the secret vehicle. There is a garage next to the motel building - open it.
From this point on, you can choose the special Disorder car after visiting any garage. Unfortunately, the game does not allow you to modify its appearance.