Mafia: Definitive Edition

For this achievement, you need to collect all five hidden cars in the ‘Free Ride’ mode of the game. You can first start working on this achievement after you complete Chapter 5, ‘Fair Game’, but will not be able to complete it until after you complete Chapter 15, ‘You Lucky Bastard’.
 
In order for the hidden cars to appear, you must first collect five post cards (even if the post card is available, the car will not appear until you actually collect the post card). Any time after you’ve completed Chapter 5, start the game in ‘Free Ride’ mode and travel to Bertone’s Autoservice in the Downtown district (just south of the Giuliani Bridge).



Enter the building and you will be greeted by Lucas who will spark up a conversation. As pay back for allowing him to help the family in Chapter 5, he is offering you locations of the five hidden cars. When the conversation is over, walk to the far wall and on the bulletin board there will be up to five flashing post cards (depending on story chapters completed). Press to pick each one up. Each one has an image of the general location the cars are in, but it is your job to find them.
 
You can view the post cards at any time by pressing then to the ‘Collectibles’ tab.
 
Hidden Car #1: Disorder (post card unlocked after completing Chapter 5, ‘Fair Play’)
Location: Vevoda Motel (North Lost Heaven, just east of Lake Fyer and the Fyer Dam)
 
Just east of Fyer Dam, there are two unmarked buildings on the map. At the northeast corner of the lower building is a yellow garage and in it you will find the hidden car.



Hidden Car #2: Lassiter V16 Appolyon (post card unlocked after completing Chapter 7, ‘Better Get Used to It’)
Location: Beech Hill (east)
 
In the eastern part of Beech Hill, along the north-south street, there is a brown garage in front of a sea green house. When you open the door, you will be confronted with the owner, so you need to first knock him out then claim the hidden car.



Hidden Car #3: Flame Spear (post card unlocked after completing Chapter 9, ‘A Trip to the Country’)
Location: Central Island (southeast)
 
Southwest of the Marshall Bridge is a fenced-in construction site. Along the south end of the site is a row of grey garages. The hidden car is in the western-most garage.



Hidden Car #4: Manta Prototype (post card unlocked after completing Chapter 12, ‘Great Deal’)
Location: Works Quarter (central)
 
Along the street that runs along the eastern side of the Central Rail Station, there is a red brick wall that can be climbed just north of the building (there is a sign that said “Wanted 8000 Laborers” on the wall). Climb over the wall and you will be in a maze of crates. Follow the maze around the crates and you will come to the back of an off-white garage (there is an iron fence along the left). Kick open the door for the hidden car.
 
If your intention is to take the car, drive it out of the garage and turn right. There is a ramp that goes over the fence, so you will need to back up a bit to gain enough speed.



Hidden Car #5: Trautenberg Sport (post card unlocked after completing Chapter 15, ‘You Lucky Bastard’)
Location: Downtown (west/southwest)
 
Along the East River, there is a section of Downtown called ‘The Waterfront’. There are no roads leading to it but you can drive a car down here if you are coming from the north. When you get to the location, there is a wall of three doors. Approach the right door and kick it in. From here, you need to shoot your way through two stacks of boxes that are blocking your path. Climbing over both stacks and walking around the boxes will reveal the hidden car.
 
To exit the garage, there are three switches on the wall to the right (if facing the door). They need to be thrown in a specific order to open the door: middle, left, right.



If you would prefer a video, here is one from PowerPyx:


Refer to The Whole Story (100G) for more information.