To get there, take the big red bridge at the northwest end of the map south towards San Fierro. Just before the intersection, there is a

small path heading down on your left. Take it down towards the diner. Once you reach the diner, you'll see a pedestal that has a mounted red piece of cable and text underneath. It says, 'Actual Piece of Cable'. Below, it says:

*Oooooh amazing

*Thats mind blowing

*Actually, no, it's simple physics and not really that impressive

Picture: http://gtadomain.gtagaming.com/images/sa/actualpiece.jpg