Sounds like a short in the clutch coil in the compressor.
Unplug compressor wiring, and with battery disconnected, check for continuity in the wiring, making sure you get no continuity when referenced to earth (one lead of the multimeter on earth)
If you get continuity to earth, you have a short in the vehicles wiring harness. Locate and rectify fault or run a new wire.
You can also use a multimeter to check resistance of the coil in the compressor. If it's a single wire (which i think it is) then one lead of the multimeter goes on that wire and one lead goes to the body of the compressor. You should see a resistance. If the resistance is zero ohms, your coil has a short in it. Replace compressor.
My money's on compressor.
Aaron.