These things can be a pain to trace. Just to give you an idea on things to look at, does the light on your radio stay on all the time? that would kill the battery over night, and it should not be on unless the radio is on. Indicates an internal problem in the radio. This will kill even a good battery overnight.. You also did not mention if it was a stock radio. If aftermarket,and a digital unit, and it has a clock in it, the clock always works, but with the ignition off, you should see absolutely no display on the radio, for instance, the clock or station feature. If you have a stock clock, disconnect it and see if the problem goes away. Same with the dome light. Often, but not always, you can disconnect the hot lead from your battery, with everything shut off, and the key out of the ignition, take the hot lead and strike it against the positive terminal of the battery (assuming this is a negative earth car). It's a quick and dirty test, but if it sparks, you've got something pulling current out of the battery, indicative of a short, many times in one of the items I've just mentioned. Hope this helps.