Diamond Motors is an auto shop in Empire Bay. In the beginning, it was designed so that Vito Scaletta could buy cars from the shop but the idea was scrapped from the game and it's no longer accessible to the player.
This business houses the Carcyclopedia inside and is seen in the opening cutscene of The Betrayal of Jimmy, where Jimmy smashes several cars there with a Baseball Bat. It can also be seen in the last cutscene of Balls and Beans where Joe Barbaro buys a brand new Smith Custom 200 in red and white.
In the mission Jefferson Provincial Theft in Jimmy's Vendetta, you'll steal a Jefferson Provincial parked in the back of the building. Sometimes cars are randomly spawned at this location and can be stolen easily.