I just recently bought a 2003 deVille off of craigslist.. The guy that sold it to me said it needed a new battery at first then he said he got it tested and it was a good battery was just dead from sitting all winter.. Well a couple days after buying it I tried to start it one day and it was dead.. Jumped it and took it to test the battery and they said it was bad so bought a new battery and the next day the battery was dead again.. Somebody told me it might be the on star system so I unhooked that and it still died over night.. The alternator is good... It has aftermarket HID headlights but not sure if that's causing it.. I just really don't know what is going on with it.. Anyone have suggestions? What is causing my battery to drain while the car is off??