Easy way to find a short. Remove the ground cable from a fully charged battery, then hook up a test light (12V) to the Ground pole on the battery and clip the test light to the ground cable (not touching the pole on the battery. I have a test light that has asharp probe on one end and a clip on the wire(CarQuest has these), and I clamp the probe to the battery pole. . If the light is on, then there is a short. Open the Fuse box and begin removing fuses until the one you remove makes the light go out. That fuse is the circuit of the drain.
My old BMW had a similar problem, the leak was somewhere in the starting circuit - if the battery dropped a little bit of voltage there was not enough power getting to the starter motor solenoid - what I did was to run a wire from the starter relay to another relay connected to the battery direct to the starter solenoid - never had any more problems
I took car to my tech with this problem and he told me I needed new battery($200).
Since problem was mis diagnosed should I be responsible for cost of battery. Btw, battery he replaced was also new which I told him about.
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