Probably the heater control solenoid. Have replaced 3 on my wives Ka. I'ts situated under the bonnet right towards the rear and central. I'ts a black plastic unit with water pipes connected to it and contains a small round silver metal solenoid. It's this solenoid that goes faulty and leaves either permanently hot or cold air blowing. Replace this entire unit with a Ford only part. The ones on Ebay last 5 minutes! Prob £30-£40
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.