battery dieing issues, need help

[Berryhill.tattooist]

I have a 2000 Bonneville ssei that ive had for a few months now. The battery seemed to die pretty easily so i bought a brand new one from autozone. One day about a week ago i got an unknown driver message on the dic. When i turned th e key the dash lights would come on but wouldbt crank over or click or do anything. The power windows and seats wouldnt work and the key fob wouldn't work. I jumped the car with cables and it fired right up. I let the car run for about a half hour and ran it around the block. Ran just fine and everything seemed fine. I parked the car in the drive way and shut it off, immediately tried to fire it back up and again it was dead and the unknown driver message was gone. I jumped it again and unhooked the battery while it was running to see if the alternator was bad, but it ran fine with the battery unhooked. I brought the battery back to have it tested and they said it was fine but almost completely dead. They charged it and i brought it back home and re installed it. The car fired right up and everything again seemed tpbe fine for about three days. Now today it seems to be dead again because its doing the same *dang* thing, wont start but.no messages on the dic. Ive searced the forums but cant seem to find a similar problem. Some posts claim to search for certain modules or bad connections, i dont know where to start. The car has hid's and an aftermarket radio. But there has been a lot of mods done to it too. Just looking for help and advice, thanks