Battery draining overnight
A neighbor had this very problem and could not find the source. As a solution he installed a battery cut off. These are inexpensive and not much trouble to install. He even mounted his so it could be manipulated from out side the car and does not require hood opening to use.. Also, if you have electric door locks, I have seen these be the culprit. Hope this helps Best of Luck.
Search Pontiac Montana Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Pontiac Montana Experts