Pucky

On my 1999 Mustang GT, I replaced the battery when it wouldn't start, luckily, in front of Autozone. A few days and miles later, I was taking her to the car wash and I watched the battery gauge start dropping slowly until it was in the red. I turned around and barely made it back to the driveway before she died. I jumped it and the battery gauge was in the middle again after unhooking the jumper cables. However, it did the same thing less than a minute later, slowly dropping until it went in the red and died again. I repeated a jump start and confirmed that the original alternator was reading only 11 vdc, as was the battery, but quickly dropped to nothing again with no major accessories running. I just changed the alternator, which read 14-ish vdc and, unfortunately, it repeated the same problem. I'm scratching my head because I've never encountered this problem before. I don't know what else to troubleshoot.