charge the battery. disconnect the negative lead, place a test light or amp meter between the negative battery terminal and the negative battery cable. if the light lights up or the amp meter shows a draw over like .1 amp start pulling fuses until the draw goes away or test lamp goes out, then you know what ckt the draw is on, go from there. It can also be a bad battery so first I would get the battery tested, or swap for a known good one.