I had the same problem with the Xterra I got rid of for my H3.
Week after week when I would drive my car on the weekends, the Xterra was dead on Monday.

Believe it or not it turned out to be a RADAR DETECTOR I had plugged into an always on lighter socket.

Yeah, I know- no way would that kind of current draw come from a radar detector, but it was the real deal. I am a master electrician and was amazed when I got rid of the detector and the problem vanished.....