hi tessa,lift the bonnet & look at fusebox,next to the fusebox in the middle of the car is a electronic valve this controls the water flow in/out to the heater matrix,this should have one hose running along side of it & another hose going into it & out again & this may need replacing,common fault on fiesta's
about £10ish at breaker's,best of luck
steray, December 2009
