You need to check all the circuits for a drain. That's done by removing each fuse, one by one, and checking its circuit with an ohm meter or test lamp. The one with a short or drain will light the test lamp or show resistance on the ohm meter. Only the clock is a legitimate draw.
Power antenna motors sometimes get stuck ON. You can disconnect its motor at the antenna - and still have radio if the antenna is left in an UP position.
Another possibility is a window motor that's ON due to a stuck switch.
Wipers seem to get a shot of power whenever the starter is used - somehow getting power from the start circuit or ignition switch. Anyone ever "work" on the car's wiring - to install a stereo or alarm? If so I'd first look there.