Interior climate control in 2003 cars with 1.6 volume of engine is efficient. During winter using of Ford Fusion heat exchanger is in operation. For change of it you need 145 mm high, 44 mm deep and 191 mm wide radiator. This is factory size of heating module for 2003 Ford Fusion with FYJA, FYJB engine. The reliability of heaters in FYJA, FYJB code motors installed in Fusion is great.
If the device is broken, you may find in 1.6 Fusion interior some lack of coolant. Also 2003 year cars of Ford may show worse heating characteristics. The best decision for Ford Fusion 1.6 is to install original heater with dimensions of 145 mm, 191 mm and 44 mm (height, width and depth).