Does leaving a multimeter on the battery posts to monitor battery voltage drain the battery at all ? Should I turn the meter off and disconnect it between checks ?
This is a 2001 Mustang, no VR to worry about.
Yes, I know the 3G alternators have an internal VR but again I know this isn't the issue.
Car can usually sit between 2 and 5 days until it wont start.
It sounds like you are on the right track. I am curious as to how much current the stereo is drawing. It may be that one of your amps is not shutting off. I have seen that a few times.I already know its the radio fuse. Pulling the 25 amp fuse stops the drain.