Many threads have been posted about excessive battery drain and what to do about it, but my recent problem solving provided an answer that didn't seem obvious and may help others.

Three years ago, I had my '73 painted. The body shop suggested replacing the switches that are in the door panel at the back of the door -- the ones that operate the "door ajar" light on the dash. The new switches were self-adjusting, a sleeve would push in when the door was closed. The problem was that they kept adjusting. Eventually, the "door ajar" light wouldn't go out. Since it was only on when the ignition was on, I wasn't too concerned. About the same time, I started going through batteries. Fortunately, I had invested in a Diehard Gold with a three year no-charge replacemnt warranty. I'm now on the fourth battery. Finally, got tired of having the "door ajar" light on. Worked with the switches, replace wiring, readjusted the sleeve, etc. Nothing seemed to work to get the light to turn off. Then I noticed that if the switch was pushed in by hand, no light. Concluded that the push rod on the switch just wasn't getting in far enough to break contact. Added a small, clear, self-stick furniture "foot" to the door panel that allowed the rod to travel further and break contact. "Door ajar" light problem solved. But it gets better. The battery no longer seems to drain overnight. Previous to the switch fix, if I didn't start it for 4-5 days, the battery charge would go below 12 volts. Since the fix, I've kept track and even after three weeks, the charge is still around 12.2 volts. Apparently, even though the light wasn't on with the ignition off, there was sufficient drain due to a completed circuit that the battery would run down. Just thought I would pass it along as another area to investigate if you're having battery run-down problems. Looks like you can't ignore any gremlins, no matter how insignificant they may seem to be. Hope this helps.

There are many thing like that in many cars. The light is off due to the ignition power being gone, but the controller for the interior lights is always powered, so the controller stayed active, trying to turn on a dead circuit.