Topper heater hose blowning out

Hello, ensure all hoses are free of cracking and dry rot. If it's over 7 years old, it should be replaced. Air should be purged out of the cooling system if it has been accessed, such as with the repair or replacement of hoses. You also did not indicate which engine your Mustang has - a 3.8L V6 was prone to cylinder head gasket failure, although by 1995 many 3.8L V6 engine's were updated in production. I would not jump to conclusion but would be aware that exhaust back pressure in the cooling system could be present, indicative of an internal engine coolant leak.