I would check the dome lights first (glove box, under hood, etc.) to see if they all turn off when closed.
Next I would check for accessories (heated seats, alarm system, etc.)
Battery drain can be measured by an ammeter in series with the Positive (+) wire to the battery. A clamp-on ammeter would be easier to use. Pull all of the fuses and put back in one at a time and measure the current draw from the battery. A small draw (mA) from the radio circuit is normal.
Don't assume that the battery itself is not the problem. My local Sears store will check it for free if you bring the battery in.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles