You need the CLEO library for this mod.

Open the "Passenger Mod.zip" file. Extract the "yo.cs" file to
the CLEO folder in your GTA SA folder.

Start the game.

Set a marker on the place where you want to go. Now, go to any car in the traffic which can seat 2 or more, and press ACTION/SECONDARY FIRE + CROUCH (default:- TAB + C). Your character enters the car AS A PASSENGER. The car drives you upto the marker, and even if you get off, will stand there under ordinary circumstances. You may abandon it and board another vehicle. Also, when you board the car, drive-by is automatically enabled. ENJOY!  

*Does not work with the Manual Transmission Mod. Also, go through the ReadMe.txt file once.

Go to http://www.youtube.com/watch?v=QUeN1rpxZq4
or view this video (with soundtrack): http://www.youtube.com/watch?v=u47nnElpCRY&feature=channel_video_title