This mod spawns street racers at night, they appear on the radar and when you drive near them and press the horn, a random destination is set and you race against the other car. You bet $200. There's 4 random cars that can appear.
It only works when you are driving a civilian car from 9 PM to 6 AM. It should work in the GTA: Liberty City mod too, I guess.

It works pretty much okay but there are still some bugs:
-If you destroy one of the street racers before challenging them to race, and then enter any car, the game crashes.
-The street racers are set to have improved handling but they are still kind of stupid 
-If you kill a street racer and then get in a car very quickly and challenge him/her, their car will drive by itself. Otherwise, the marker will dissappear but it takes a few seconds for some reason.
-If you are too far away from the street racer, you may win even if you were losing. I think this is because the opponent must have crashed and got killed.