I had the same problem recently with my 2000 RX300. When it got cold, suddenly my battery would drain completely. My husband checked the battery to make sure that was not the problem, using his digital mutlimeter. He checked to verify it was not the alternator either, and then proceeded to check each fuse to find where the drain is occurring.
He has isolated the problem to two specific fuses, 15 and 16. One controls the dome light and interior lights, and the other one controls the ECU. Based on everything I've read so far, here and other web sites, I'm thinking it has something to do with the door(s). The reason I think that is that we have a stubborn lab that refused to get out of the car one night. We left the door open (after turning off the dome light) for only about 10 minutes before he finally decided to join us in the house. The battery was dead. I usually drive the car by myself, and other than open the rear hatch, rarely use the other doors for anything. Almost every time I use that same door (backseat, driver's side), the battery dies.
We are going to assume it has something to do with that door. We're going to try the door lock mechanism and anything else connected to that door. 'm tired of recharging my battery. The weird thing is that it runs fine for weeks, but then suddenly it starts having issues again. At first, we thought it was the cold weather, but then it seemed connected more and more to whenever I had the occasion to use that back door.
I'll let you know if we find out anything.
Keri
