There have been lots of posts on the subject of draining battery, like maybe a million, OK slight exaggeration, but if you search on "draining battery" I think you will find lots of posts, some of them quite lengthy with lots of ideas on what to check. This is definitely a recurring theme. Off the top of my head I remember a few of the many possible causes: Trunk light on all the time, bad voltage regulator, bad alternator diodes, aftermarket alarm, radio malfunction. There were others also. Give search a try.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra