Before I start, this is less for the benefit of fellow modders, but more for those who are avid GTA players but hate the lousy control setup in San Andreas.
These extra buttons drastically improve the ability to do drive-bys and fly in GTA:SA. The one next to the left trigger is the white button, and the one next to the right trigger is the black button. Yes, I know thet aren't exactly symmetrical, but they work for me and this wasn't intended to go in a museum.
I got the pinout from jeremydammit who did a hell of a job finding and labling the pinouts for each button on the s-controller. Check out his thread here: http://forums.xbox-scene.com/index.php?showtopic=301912
REMEMBER THAT THE BLACK AND WHITE PINOUTS ON HIS DIAGRAM SHOULD BE REVERSED!!!
I first did this with some buttons from radio shack. It worked fine, but I didn't like the tactile feel of the RS buttons, so I ordered other buttons from mouser.com and they feel much better, as well as contribute to a more finished look for this mod.
While some may view this as cheating, (like the controversial Halo crouch switch) you may want to remember that the PS2 controller has two pairs of shoulder buttons which allow for excellent control in GTA:SA, but the layout of the S-controller doesn't translate well in that respect. In reality, this layout reflects the intended feel for the control set, and is a little closer to the 360 layout.
Hope this helps GTA fans enjoy the game a little more!