sdl battery drain

[kenwc21]

Well my beautiful 86 sdl is quickly becoming my little problem child. I am currently not driving the car, waiting on transmission repairs, or rebuild. So the car sits.

I have very little info from previous owner so I am blind as to its history or repairs.

I have installed 2 different batteries (not cheap ones) in the last 2 months. Today when attempting to start the car, the battery is COMPLETELY DEAD. No lights, no radio, not even the ignition buzzer. My mechanic has d/c the alarm, both horn and lights. What could possibly be causeing this problem. Not the alternator, I'm not driving the car enough to re-charge the battery. Thanks for the suggestions and help.