Turn the amp OFF if it has an off, and disconnect the inputs and outputs so that the power is the only connection to the amp. Does the fuse still blow? If NO, does it blow when turned to ON?
If still NO, add back the inputs one at a time and repeat the above steps.
If still NO, then add back the outputs one at a time.
This is pretty basic in diagnosing a short without ruining a 12v meter.
What is the rating of the fuse you are using inline on the 12v lead?
Have you added a humongous capacitor or any other accessories to the amp?