The trunk latch / switch for the trunk lamp has been known to break. This causes the lamp to remain on when the trunk is closed. Since you can't see it, it goes un-noticed. It will draw the battery down in 2 to 3 days. The best way to find a problem is to measure how much current is being drawn with everything off. That is, key out. Only a few components in the alarm system and remote door lock system are still "alive". Typical draw is not more than 1/2 amp. Don't dismiss the possiblity that your original battery was bad and was self-discharging. The battery buddy is a good idea and I believe a standard feature on most new cars. If you aren't comfortable with electrical work, a shop check of the entire charging system might be in order. It's a long shot, but a leaky diode in the alternator would also cause a drain as well as a failed component in the voltage regulator. (Those diodes leak electrons, not water.)

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Resistance is Futile.