If you have a problem with the battery going dead overnight you may have "voltage draw". To locate the circuit with the problem you can disconnect the negative battery cable and hook a test light up, one end to the negative cable and the other end clamp to the negative battery post. Make sure everything in the car is turned off, including the interior lights that will cause voltage draw when the doors are opened. The interior lights won't light up once the test light is hooked up as instructed above, but if not turned off they will light up the test light giving you a false indication of a voltage draw. With all the accessories turned off and all the lights off, if the test light comes on bright you have voltage draw that is draining the battery. If the light is very dim, it may not be a problem and could just be the ECU voltage consumption. If the test light is on bright you now can pull the fuses one at a time to see which one makes the test light go out. Then you will have found which circuit has the voltage draw. From that point you must have a wiring diagram so that you can figure out what is running off that fuse which may be the problem. This is not an easy thing to do and many mechanics are not good at it.
