My 2005 Forester's battery drains slowly over several weeks. I put a slow charger on it to recharge, but it continues to slowly drain its charge. I read that the car has certain electrical functions with its computer that continue working even while not running. However my 2010 Forester doesn't have this same problem, so I suspect there is some short somewhere in the system that is causing a greater drain on the battery's charge. Any suggestions on how to diagnose this issue or how to locate a short?