First thing to do is charge the battery, and verify full charge with a meter, around 12.8 volts or so. Then disconnect the ground cable and go away for 2 days. Come back and check the battery voltage, should be over 12 volts. If not, it's battery time. If it is, wait till almost dark to hook it up. When you connect the ground wire, previously removed, pay attention to how big a spark you get. If it is large, you have a problem. If it is small or none, connect and tighten. Now go in and pull all the 12 volt fuses, making note of what came from where. Now wait another day and check the battery. If it is drained, then it is something hardwired. Slide, gas detector, other stuff.
There is a faster way, with a a voltmeter. Obtain a 1 ohm, 5 watt resistor. (radio shack maybe). Connect one end of the resistor to the negative lead. Connect the other end to the negative post. Now measure across the resistor with your voltmeter, negative lead to negative post on the battery. Use the 2 volt scale. I won't get in to the math, but if you read 1.2 volts. for instance, that means you r are pulling 1.2 amps out of your battery. (the math works by having a 1 ohm resistor). You can start pulling circuits and checking fuses until the voltage reading is very low, .1 volts or less.
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