Please help, I'm completely confused
 

So I have a 2004 Nissan quest. When I bought it the radio, DVD player, odometer and a few other electronic things didn't work. No big deal however if you didn't drive the vehicle regularly it would kill the battery from whatever was going on. I didn't have that problem as I use it as a daily driver and I've had it for about 6 months with no other problems and it hasn't died since I do use it everyday. Just last week I noticed the battery light started coming on when I was driving it but after a while it would go off. Yesterday I started it up and turned the defrost on to warm it up before I drove it and when I went back out it was dead. I had it jumped and it started up but didn't have power and it shut itself back off. It won't even take a jump now to even move it to a garage. I am a backyard mechanic so def not a qualified technician but I have enough experience to think it was the alternator the way it was acting as it doesn't run on the battery but just uses it to start however after about 6 hours of fighting to get the alternator out and having it tested I found out that's not the problem. It tested good. Can anyone shed some light on what I should try next or where to even start?