The Mount Chiliad Ghost Car is a proven myth in GTA San Andreas. It has to do with a vehicle that drives down the bike track on Mount Chiliad. It is a proven phenomenon within the game.
If you approach the bike track on Mount Chiliad from the east, you will sometimes be able to observe a vehicle driving smoothly down the track. The car will act as if it is driving on a normal road. The car isn't actually a "Ghost Car", as it is just a random vehicle being driven by a pedestrian. The vehicle is strange because cars are not normally supposed to spawn on the bike track. Also, the car exhibits a few strange properties. Since there are jumps and wooden boardwalks along the path, the vehicle will warp through them, as if they are not there.
ExplanationIt is believed that the Mount Chiliad Ghost Car is simply the result of a vehicle spawn glitch. Vehicles aren't normally supposed to spawn driving down the bike trail, but there are path files for the trail that exist in the game. These path files are used by the bikes in the Mount Chiliad bike racing side-missions. It is possible that a random vehicle will accidentally spawn there because of a glitch, and will follow the path of the race.
It acts like just any other vehicle, and is already running when you approach, as if it was coming from somewhere - but as it works in the game, vehicles and characters just spawns within a certain range of you, so they don't actually come from anywhere.
It is difficult to witness this glitch, but with some luck and perseverance, the player may be able to spot the Ghost Car. The easiest method of exploiting this glitch is to obtain a Jetpack and fly towards the mountain from the east. The player will just have to repeat this over and over again until a vehicle is spotted driving down the path. As noted before, the vehicle will be a random countryside vehicle.