you got a Volt Meter?
Put the battery in, run the car up , make sure its charging at 13.7 or so volts.
Switch off
keep the volt meter on the battery and see if the voltage is being sapped ( a good 12v batt should sit at about 12.4 volts just sitting in the car) if it starts to drop , then its a process of elimination.
You can try simple things like removing fuse 1 at a time and rechecking the voltage and see if it stops dropping.
Remove you stereo - does it stop dropping?
Disconnect the plug from the Alternator - does it stop ?
etc