its very likely to be the heater control valve that is at fault, it's under the bonnet just tucked behind the false scuttle panel in the middle, you should see 2 hoses connected to it from the front side of the false panel.

you will have to check on ford's latest revision of this as there are so many versions and some are known to cure the problem and some are not.

think the part no. is relevent and is back to ending in AA again, not AC as per the last revision.