You will have to connect a multimeter set to the 10 amp scale between the disconnected negative battery cable and the negative battery terminal.
The reading on you car with everrything off and having set for several minutes should be around 30 milliamps, any more and you will need to start plucking fuses from the under hood fuse panel one at a time until you see the reading drop back to 30 ma.
When this happens, you have located the AREA that is drawing the amps.
Go from there to resolve the problem.