Yank fuse 8 and 23 (if it is an 05) and you will have the radio powered down completely. If the battery does not drain after that, it's either the radio (fuse 8) or the power amp (fuse 23).

The faster way to find out is with an ammeter inserted in series with the battery ground cable - that will tell you what current is flowing and if it's too high, you can pull fuses until you find the one that is the key.
If NO fuse is the cause, you would then look at the heavy lead going to the alternator - a leaky diode in the alternator causes this as well.

Remember to raise hood, turn key off, remove key, close doors and trunk - and wait 180 seconds after connecting the meter (for modules to hibernate) - then, any current over 50ma (.o5 amp) is too much.