You can Google "locating auto parasitic draws" and get a ton of info.
Quick and dirty, disconnect the battery cable from the battery's negative terminal. Fully charge the battery.
Now, attach a 12 volt test light in series with the battery cable and the battery's negative terminal.
Now, for a little background. Modern vehicles will ALWAYS have a slight parasitic draw. The PCM, the radio memory presets, that kind of stuff. That draw should be less than 50 mAmps, more like under 30 mAmps.
Ok, with a NORMAL parasitic draw of 50 mAmps or less the 12 volt test light placed in series with the negative cable and negative battery post will only have a DIM GLOW.
IF THE LIGHT GLOWS BRIGHTLY the parasitic draw is excessive and is killing your battery.
You can also use a multimeter to measure the parasitic draw, most meters have a 10 amp max draw circuit. However, the 12 volt test light is better BECAUSE of what you will find out next.
IF the parasitic draw is too high, you need to locate the circuit with the problem, then the part of the circuit--wiring harness or component--that is the specific cause of the problem. WITH THE 12 VOLT TEST LIGHT GLOWING BRIGHTLY it is quite easy to pull a fuse, look at the light, no change then that circuit isn't the problem. Keep pulling fuses until the light goes dim. IF YOU USE A METER, EACH TIME YOU PULL A FUSE YOU HAVE TO RUN OVER AND LOOK AT THE METER'S DISPLAY.
Keep pulling fuses one at a time until you locate the circuit. Note that you may not find the problem just by pulling fuses, you may have to disconnect harness connectors and/or components--a/c, aux fan, blower motor, etc., to nail things down. You can even have something behind a fuse block or under the PDC causing the problem.
Good luck.