When you hook up the positive cable on your car to your battery, you’re hooking up electricity to a network of wires that are isolated from the rest of the car. When you turn something on in the car, the circuit is completed and electricity flows thru the entire system making whateve you turned on work. When one of these wires “shorts,” which is the cause of battery drains, that simply means that one of the “isolated” positive wires is coming into contact somewhere with the rest of the car, or the negative side of the car. When this happens, you actually have positive electricity on the “negative” side of the car, the “negative” side being the body of the car and everything else that isn’t an “isolated” positive wire. The “trick” Eric described above (inserting a 12 volt light that you can buy most anywhere for $5 ) between the negative battery terminal and the negative battery cable) tells you if a positive wire is touching somewhere it shouldn’t. If the light lights, then you have positive electricity on the negative side, or an electrical “short,” since the light should not be in a complete circuit and thus light, if there’s no positive energy on the negative side of the car. If the light lights, then you know there’s a short and it becomes a matter of finding which wire.