Check the battery, alternator, and battery cables. If they all check out then check the grounds. Then add a few more grounds (body/frame, frame/engine, body/engine). Then sit in the car with the door closed the windows up and no key in the ignition. You may be able to see or hear anything running that's draining the battery. Also check to make sure the fuses are the correct size and the relays and curcuit breakers are present and the correct type.