The Unused Police Car is a car that was cut from Midnight Club II. It is only mentioned in the texture files, and does not appear in the finished game. It can only recovered by using mods such as MODern Edition or Midnight Club II: Decade Edition modification made by TEK Vito
(from LCPDCV on IGCD)
It's likely that Rockstar Games didn't have enough time to add this car to the final game.