Intermittents are the toughest problems to diagnose and correct. If your battery holds its charge while in the current configuration ( shell parts removed ) and the drain returns when you replace them, it implies one of those now removed parts, when in place, is completing the high resistance short that drains your battery.
I'd suggest you carefully examine those parts as you replace them to see if they fit without touching anything in the electrical system (monitor the path that is now correctly reading 0 volts drop while replacing the parts. If it suddenly changes from 0 volts as you reinstall the shell parts, the one you touched last when it changes is causing your problem. There may even be evidence of the intermittent connection path on the inside of one of those parts. Look closely.
joe