First thing you need to get is an ammeter. If you have a DMM with a 10 amp scale, that'll do nicely. Then proceed like this...
If you have a capacitor in your audio system, remove it.
Verify that there is indeed current being drawn from the battery when everything is supposed to be 'off'. Do this by putting the ammeter in series with the battery positive terminal and the big red wire. By your description, I'd expect to see a half amp or more being drawn by something.
Start pulling fuses fom your fuse panel (and stereo wires) one at a time, while watching the ammeter. If you get lucky and one of the fuses makes the current go away, then you'll need to know what items are on that fuse, and track it down.
Don't be alarmed by small sparks when you insert the ammeter in circuit. There are capacitors in the radio and your car's computer that will spark when they charge. You're looking for steady-state current draw. You can ignore the measly couple of milliamps drawn by your clock.
Places to look are lights in the trunk or under the hood that could be on without you seeing them, or chaffed wires in your wiring harness.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's