The Unmarked Cruiser is a police sedan based on the Vapid Stanier in Grand Theft Auto V. Little is known about it other than its existence via a page in the Rockstar Games Social Club website. It can be found in numerous color types, medium gray apparently being the most common.
The vehicle is based on the second-generation Vapid Stanier, much like the Police Cruiser, Sheriff's Cruiser, and Taxi. It is in several colors with black trim and has no exterior markings, which is suppossed to make it harder to be detected as a police vehicle by civilians and to offer some degree of anonymity to officers who need it, such as Detectives. According to the RGSC website, the Unmarked Cruiser has similar performance to its marked siblings, which means a high top speed with average handling, poor braking, and average acceleration.
Obtaining itTo obtain it you must do Barry's Weed Stash missions which is around.
Go to the highlighted green marker on map (Northeast of Los Santos)
Kill the cops and save it at a safehouse or garage.