You need a basic multimeter set on amps. You need to put this into the battery circuit. So remove the positive battery wires and conect the multimeter up in between those wires and the battery. You should see less than 0.5amps of current (make usre your doors are shut so the interior light is not on etc). If there is more than 05v amps remove a fuse, if it drops the fault is in that circuit. If it stays the same, replace the fuse and try another one and so on.