How to find battery drain??

[King]

Every winter my X5 seems to mysteriously die. I went to start it in the morning and nothing...not even a click! Last year I replaced the battery, so it's only a year old. Truck was even parked in the garage and still won't start. I charged the battery overnight and it started right off the bat!

It's got to be a battery drain somewhere, but where the heck do I even start to troubleshoot something like that?? I have a feeling it's the damn trunk latch, although I never got the warning in the dash! This is really driving my mad. Several times I get ready to go to work in the morning and the damn thing won't start!