Let's face it, nobody wants to walk out to their car, hit the key and hear the dreaded "click click click" of having a dead battery. It's even MORE dreaded after they've spent over $200 on a brand new top of the line battery either.
I've been asked about this topic more times than I care to admit to, but if you find yourself in either of these situations, you've more than likely got the dreaded short somewhere in your electrical system that is killing your battery. Granted, cars will always have a current draw on them from SOMETHING. The ECM, clock on the radio, something. But those items aren't enough to kill your battery unless the car has sat idle for MONTHS on end without starting it.
So, you want to find a short, but don't even know where to begin? Here's what you do:
1: Remove the NEGATIVE cable from your battery, and install a test light from the negative terminal to the negative cable. If you've got anything drawing power, the light will come on.
2: Remove anything from the equation that has been added on such as security systems that run directly to the battery, or cables to amplifiers, anything aftermarket that has been added and wired directly to a hot 12V source. You want to eliminate those and get down to the nitty gritty of the car itself and the factory wiring. Every time you remove one of these add ons, check the test light. Does it grow dimmer? If it does, you've got something you've added to the car drawing power. Make a note of what you removed, the lights action, and so forth. Some of this stuff, like an alarm system, will draw power, and you're not going to get away from it. You're just looking for the BIGGEST draw, at this time.
3: Once all the "added" stuff has been removed from the equation, it's time to move onto the cars systems. All circuits in an automotive system are fused. Since we know this, checking which circuit becomes easy once we realize that pulling a fuse is essentially shutting off that circuit. So now we go back to our test light and see that it's still glowing. We move on to our fuse panel and remove each fuse, one at a time, then checking the test light to see if there is any difference in the light's brilliance. If the light goes dim after pulling a fuse, then you've just found a circuit that's shorted! Keep in mind the dome light and clock circuit is drawing as you do these test! Take them out of the equation by removing those fuses first!
4: After we know what circuit is shorted, we need to locate the wires that are in the faulty circuit. This is where a factory wiring diagram is invaluable!
(Start of Heath's rant) Don't rely on the one's in Haynes manuals, or Chiltons. I've come to find that sometimes they denote a wire a certain color only to have it a different color on the car. Trust me on this and source a factory diagram! It's not worth the headache trying to decipher how Hayne's or Chilton screwed up while trying to figure out your OWN problem. (Rant over)
Physically look over the wiring. Check the insulation. Is it burned? Is it rubbed through? Is the wiring intact? Has someone made a faulty connection somewhere that's rearing it's ugly head? You'd be surprised the amount of times I've found shorts just due to abrasions on the insulation, or somebody that didn't know what they were doing playing "Hack and Tack" with wiring in the factory harnesses.
5: If you can't find any physical damage, then you need to have a digital multimeter (such as a Fluke) and start checking continuity between individual wires and ground. Again, the factory diagram is an invaluable tool for this. Take the test light out of the equation. This will only complete a circuit, and we don't want to do that at this time. You obviously will have continuity on ground wires, but you shouldn't have any on hot wires after the test light is removed. NOW start checking continuity on your hot wires in the circuit in question and ground. If your meter gives a beep, you've just found the shorted wire. Chances are good that you'll end up finding an abrasion or some other abnormality that you didn't see before.
Usually you can find dead shorts by just watching your test light as you pull fuses, and physical inspection afterward. But sometimes you have to dig into things with a meter to find out what's going on where. Nobody hates wiring problems more than me. In over 20+ years of turning wrenches on things, I've seen a lot, thinking I've seen it all until I get one with a new issue. Wiring issues can seem daunting, but with the right equipment, and proper troubleshooting, you can eliminate a lot of stress in finding dead shorts and electrical issues.
Hope this helps!