What causes my battery to die over night?

Hello, likely one or two Seat control def. You can try to disconnect them over night. The plug is in front of the seat underneath(square plug 4 pin 3 cables) disconnect left and right to make sure. Check next morning if car starts.

sounds like you are leaving the key in the ignition - that keeps the car doing all sorts of things and the battery will be drained overnight - make sure the key is removed from the ignition

Battery control module is probably the problem. I'm sure you have taken care of the problem but I wanted to get the info out for those who are facing this problem. The BCM cuts off the power to all the accessories some minutes after removing the key from the ignition. If it fails, the car will continue to function as if the key was still in the on position as far as the accessories are concerned. It's the little black box located in the trunk near the battery negative connector cable.