1994 Buick Lesabre I have no Idea what's wrong!

When it rains overnight, my car will act like the battery is dead the next morning, and will only hold a charge for 2 hours at most. I took it to a local auto parts store, and the charging system checks out fine. The battery is 10 months old, the alternator is 6 months old, and the starter runs fine.

When I took it to a repair shop, they couldn't find out what was wrong with it either. I suspect there's a short in the system somewhere, so I've been disconnecting and reconnecting the battery to avoid having to jump it three times a day.

Thanks!