If it is a recent problem I would look a any new additions to the electrical system, any new stereo gear, lights, or any other devices. check to make sure none of these are powered up when the key is off. I had an amp in a friends car that kept killing his battery. then check for any obviously broken wires or connectors that may be shorting to ground. Problem is there are litterally thousands of wires in the car and anyone may be the problem. Also any device that is powered when the car is off that is bad or shorted could cause the problem, look at areas of the car that are not working properly first. If the rest of the car works and you can't find it try a battery disconnect switch that can be turned off at night to preserve the battery. good luck

 

might try the battery disconnect while you are searching. some are available that connect at the battery and the original cable connects to the switch, also a good theft deterant.