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An electrical short will do that. Disconnect the + battery cable. Shut the ignition off and have the doors closed. Nothing should be drawing any current from the battery. If you have an electric clock, then unplug it. Put a test light between the battery post and the disconnected battery cable. If it lights up, then you have a short circuit somewhere.

Now you have to find which circuit has the short. That's a pain, but can be done with a test light one circuit at a time again with ecerything turned off.. One of the circuits will be drawing juice when it's not supposed to be. When you determine which circuit it is, then you can start to trace down that circuit until you find the frayed wire or faulty connection or faulty component that is creating the short.

Thats all assuming that it is a short causing the problem.

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