Step 9 - If no other electrical accessory is causing the battery to drain overnight a manual draw check of the electrical system will need to be performed. What this means is you will be checking the electrical draw the battery has on it when the car is locked up, with the key in the "off" position". First open the hood and disable the under hood illumination light, if equipped. Next, with the key off and the doors locked wait 15 minutes, then disconnect the battery cable on the negative side. (The 15 minute wait allows the computers to go into "sleep mode" and shuts down all electrical).
Attach a test light between the negative battery cable end and the negative battery terminal. The test light should illuminate dimly or not at all. If the test light is on brightly there is a strong electrical draw in the system. To locate this electrical draw start removing fuses one at a time. When the test light goes out the circuit in question has been located. You will need a car repair manual to identify all accessories in a particular circuit, repair as needed and re-check system.