Derelict cars are important, due to their ability to be upgraded to the maximum level without the level caps that your regular cars have. After upgrading a Derelict car to Performance Level 300, a Super Build will unlock for even more visual customization. After restoring a Derelict car, you can also choose which category you want it to be: Race, Off-Road, Runner, Drag or Drift.
You'll need to collect the five hidden Derelict car parts for a given derelict car to unlock it, and each derelict car requires you to have defeated a different boss. Once a league boss has been defeated, you will receive clues for the five derelict car part locations of a certain car, which you can find in the "Derelict" tab. It shows a drawing with outlines of the streets where the Derelict parts are located, and parts will not spawn until you have defeated the corresponding league boss. Assembling a Derelict car also unlocks the "One Man's Trash" trophy, and if you upgrade one to level 399, you receive the "Another Man's Treasure" trophy.
Chevrolet Bel Air 1955- Beat Runner questline
Chevrolet C10 Stepside Pickup 1965- Defeat Riot Club Boss