2002 Galant V6 Battery Not Charging

I have replaced the alternator and battery and still doesnt hold a charge. The car started when new battery was installed then while driving lights will dim, all the gauges will start flickering, the engine will start hesitating and then eventually die. It killed a new battery in 1/2 an hour. After changing the the alternator I had the original tested and it passed but i left the new one in. I pulled every fuse in the dashboard and under the hood and none of them were blown. I also tried tracing the harness from one end to another but couldnt see the whole thing because some it is attached to the side of the engine and is tightly wrapped. I have a stereo system in the car since I bought the car in 2002 but i disconnected the main power from the battery to see if that was problem but the car still doesnt charge the battery.