The Car Dealership/Showroom mod in Grand Theft Auto 5 allows the players to introduce an entirely new element of gameplay dynamics in the game world. This mod introduces a fully fledged car dealership into the game where players can walk in and purchase new vehicles, just like in real life. Players are able to explore a variety of cars on display in the showroom, and information about each vehicle such as model, year, and manufacturer gets displayed when the player approaches a particular car. The characters can interact with the salespeople as well. The mod also offers the chance to customize the cars' appearances, from color to additional features. This mod aims to increase the level of realism and immersion within the game.

singleplayer installation guide-

Extract the YMAP from the folder and set it on your desktop
Open OpenIV, install it if you dont have it already
[Make sure you go into edit mode!!] Click update> x64> dlcpacks> custom_maps>dlc.rpf> x64> levels> gta5> citye> maps> custom_maps.rpf
Then take the YMAP and drag & drop it into OpenIV then its in!

Credits:
Dylan-