1996 Lexus ES 300 Battery Drain

Dear Mechanics,
My Lexas ES300 is draining the battery. My mechanic detected a 20mA of electric power flow, but could not find out why and he told me the alternator looks ok. I have changed 4 brand new batteries in the last 6 weeks. One of it even could not be recharged. Start from last week, I disconnect the positive end of the fully charged battery whenever I do not use the car. It should avoid all the possible power drain. Now, again the car will not start. It looks like an alternator problem, but the symptom is kind of wired. What happened is after I connected the battery, all the door lights were very bright, after I started the car, the panel flashed once, and turn dark immediately. All lights were gone. The car appears totally dead now. If the power is less than 12V, the lights are just dimmer. I suspect something wrong with the electric circuit and short the power line somewhere. Two years ago, I forgot to close my sunroof, rain flooded the car from inside. I did not notice it and start the car. I can hear very loud like explosion sound coming from the sound system inside the door. However, the car runs fine for two years. If the circuit gone bad now, will it short the battery(one of the new battery could not be recharged)? Do you think it is the electric circuit problem or the alternator problem. I really appreciate your comments.
Sincerely,
Alice