To DIY...you'll need a DC ammeter to measure the current draw. A clamp-on DC ammeter starts at about $25. Clamp around the negative battery cable and see what the current flow is. As jos wrote, it may take something like turning the ignition on then off, or whatever makes it happen on your car. When you see the current draw start pulling fuses. One by one pull then replace the fuse. Does the ammeter reading change? If necessary, take out all the fuses--make good notes of which value fuse goes into which slot; it's already marked, so pay attention. If you have all the fuses out and you still see a draw---it's time for a pro. The rear window defrost heater has a fuse, and this would have found the problem there but not the cause.