you probably have something draining the battery. check to see if maybe the trunk light is staying on or something of that nature. but more than likely if the new battery is dying too than someting is still on.
Hook up a multimeter set to amps, trick the trunk into thinking it's closed, let the car have 15mins (all major electronics will fall asleep) check your amp draw... 0.5amps is considered normal. If yours is higher after 15-16min start pulling fused ONE AT A TIME until the draw drops within the normal range
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