Help! Slow Battery Discharge
Also, any car (especially that old) that is in a wet/ corrosive/ humid environment, check such things as
The routing of the rear harness, which can suffer moisture damage.
Door switches and the lighter socket, which can accumulate wet sludge/ rust/ corrosion that can be conductive. This may not cause much of a drain, but they can cause SOME
It might even be possible that moisture / corrosion has "built" a conductive bridge up in the ammeter