Before opening and during all repair of iPhone 3GS make sure it is OFF. As you know #6 is a simple snap top connector. You need to look very carefully that the top is directly over the bottom logic board portion from all angles before you gently but firmly press it down. The only problem I can see is if the parts are not damaged, is that the cable tends to be about half a mm to long. So as you carefully line it up, push up under the cable with a paper clip or small screwdriver to have the top connector (attached to flex cable) pulled back that half mm so it lines up perfectly, then press it down.