One method that works is to:
1. Turn off the ignition
2. Disconnect the battery positive side terminal lead
3. Wire or alligator clip a 12v bulb between (in series with) the positive terminal of the battery and the now disconnected positive terminal lead.
4. If there is any light indicated by the bulb, you know something is drawing current. You probably should disconnect the radio memory to start out. That will draw some power, but it should be very little.
5. Start disconnecting things one-by-one until the bulb goes out
