Vauxhall Meriva fan stays on when engine is turned off

The cooling fan stays on even when i turn off the ignition[ BUT GOES FROM HIGH SPEED TO LOW SPEED] and flattens the battery. It comes on as soon as the engine starts .There is a coil that plugs into the rad plastic cover and it gets very hot as soon as I turn on the ignition. If taken out and unplugged the fan only works when engine is running, and goes off when the ignition is switch off any ideas as to what the coil is called I cant find it on parts lists for my car HELP PLEASE