A case fan only draws about 250 mA, so this is easy. Strip back the red and black wires about 1/2 inch. Grab one of the large drive power connectors. The 12V wires are black and yellow. Use a razor or utility knife to (gently) scrape off some insulation; run the blade parallel to the wire to expose the conductors (it helps to bend the wire slightly and scrape along the outside of the curve). Wrap the stripped fan wires around the power conductors black-to-black and red-to-yellow and tape them up. That should do it.