Here is a quick trick. Disconnect your negative battery terminal. Use multimeter set to "volts" set to read up to 12V (use a cheap one in case you get your polarity backwards) and insert this in series with the disconnected battery cable. It has been a while since I have had to do this, but I think you should conncet the negative voltmeter lead to the loose negative battery wire, and connect the positive lead of the volt meter to the negative terminal of the battery. Make sure all systems are off. Voltage reading should be very low. Turn on the ignition as a test and voltage should read 12V. Shut ignition off and close door to eliminate dome light draw. If you have significant voltage reading (anything above 1/2 volt would be a concern I think, start pulling fuses until it drops off suddenly. Now you have the circuit identified and you can start figuring out where the exact problem is.
Becareful, safety first! A 12V battery won't shock you, but if you accidentally ground it against the car frame or with jewekly or tools, it will burn you or the car and/or the garage/house! Always disconnect the negative cable first to remove the ground circuit and then the positive if needed. renove rings watches and necklaces. Don't laugh, but have a hefty pair of dykes to disconnect the battery in an emergeny and have a fire exteinguisher handing as well.
Hope this helps!