Disconnect the negative batter cable. Connect a test light between the negative post and the negative cable. The light will be on as long as something is drawing power..such as an open door or ignition being on. Is all sytems are off and the light is on then you have a short. You can begin removing fuses until the light goes off....this will help locate the shorted circut. After that you'll have to begin checking the wiring.
I assume your battery does not have an internal short???