What could most likely cause the battery power to drain? on 1998 Cadillac Seville STS

All is well when the car is running.Once it is parked,overnight, the battery goes dead. Where do I start to look for the problen?

Asked by for the 1998 Cadillac Seville STS
Start with checking the condition of the battery, be sure it is fully charged. Cables clean and tight. Also have starter and alternator checked, they might have a short that is draining the battery. After that if you haven't found the problem, you can take the negative cable off the battery and attach a test light to the neg cable end and the other to the car body if the light comes on you have a short. Remove fuses one at a time until the light goes off. Then check everything on that circuit, that is where your short is.
1 more answer
Start your car and disconnect the positive wire from your battery. If your car dies, it is most likely your alternator.