2000 Ford Mustang

You have a black tube that comes out of the water pump and runs back to the rear of the engine and goes to the heater core. There is a bolt on the top of this tube right above the water pump. Start the engine and remove this bolt to bleed this line to get the circulation back to the heater core wait till you get a steady flow out of the bolt hole before tightening back down this should fix your problem, it did mine