Alternator problems

I have a 2001 blazer, it was giving me problems, had it tested said alternator was bad, had that replaced and the battery also. Ran fine for about a month, now it is doing the same thing again. Went to start it, no clicking no nothing, put it on charge, it started but now will only stay running as long as it is in park. After I charged it, the gate ajar light came on as it did before, and the window popped open before, this time it didn't but had to open it and shut it and light went off. Had no signs of any problems, came home from work, 6 hours later, it was dead