99blazer charging problems

A month ago my battery light came on while I was driving and I called a friend who is a mechanic about this problem. He told me that I needed a new alternator. So I got that put in my truck. One week later my battery light comes on again and it was showing that my battery was dying so I bought a new battery. A week later my battery light comes on again and I took it back to my mechanic and they ordered me another alternator (thinking maybe the other one was a bad one). My truck was fine for a week again and my batter light came on again and completly broke down. I got my truck towed and they put new wires in from the battery to the starter it is now another week and my batter light again has come on and it's losing it's juice. Can anyone help me?