Ford 2003 Mustang Workshop Manual page 1520

Remove the coolant bypass studs.
37. NOTE: LH is shown, RH is similar.
Remove the bolts and the coolant bypass tube.
38. Remove the bolts and the heater water inlet tube.
39. CAUTION: Do not use metal scrapers, wire brushes, power abrasive discs or other
abrasive means to clean the sealing surfaces. These tools cause scratches and gouges
which make leak paths. Use a plastic scraping tool to remove all traces of old sealant.
NOTE: LH is shown, RH is similar.
Remove the fasteners and the LH and RH valve covers.
Remove and discard the gasket, clean and inspect the sealing surfaces.