Before you can start collecting the hidden cars you need to have beaten all 20 chapters in the story. Once you’ve done that launch Free Ride and go to Bertone’s Autoservice that’s just south of North Park on the map.
Once here you will have a brief cutscene, walk over to the notice board of the right side of the room and interact with all 5 post cards. After collecting the 5 posts cards from the wall you are free to start collecting the hidden vehicles.
If you prefer a map, check out the map created by PowerPyx here.
Trautenberg Sport
00:45 - This can be found in the Waterfront area that’s just south west of Downtown. Once here you will have to kick down a door and shoot the boxes that are in your way. Make your way through all the boxes to reach the hidden car. To open the garage you need to press the middle switch, the left one and the right one in that order.
Flame Spear
02:01 - This can be found in a garage that’s south of the Central Island on the map. Look for a building that’s the shape of the letter “J”. The garage is in front of that building.
Manta Prototype
02:28 - This can be found just south of the words “Works Quarter” on the map. Go south of the first “R” in quarter past the building. You then need to jump over the wall and navigate your way through all the boxes. Eventually you’ll see a garage door where the car will be.
Disorder
03:17 - This can be found east of the words”Fyer Dam” on the map, you will see a sign that says “VEVODA”. Open the garage door to get the car.
Lassiter V16 Appolyon
03:49 - This can be found just north of the Beech Hill on the map. It’s in the garage that’s just north of the first “L” in hill.
If you are struggling, feel free to use this video as a reference.