Obviously you have a drain on the battery even though the key is turned off. Using an amp meter try disconnecting one wire at a time to find where the drain is coming from, when the particular wire is located, you can begin the tedious job of tracing where it goes and thus finding what is causing the battery drain. Not easy by any means. Try and get a good wiring diagram before you start, it may simplify your life.
Have the battery charged and remove a cable and let it sit all night and see if its discharged??? If so the battery itself must be bad, a shop will test it for free. If it stays charged if disconnected but runs down when connected, there must be a parasitic current draw but where??? Maybe try removing fuses one at a time to see which circuit is drawing the current??? If a circuit is drawing very much current you will get small sparks when the fuses are removed or plugged back in. An in line ammeter can measure the current draw.