Is your Mustang Heater Core leaking? 2001-2004 Mustangs are notorious for having problems with the heater core leaking. Some symptoms of a leaking heater core are a frequently steamed up windshield, strong coolant smell in the car, and constant wet floor in the passenger side floor. Although it is a tough job, the reward will be worth it. Replace your broken Mustang Heater Core with a quality replacement and rid yourself of coolant soaked carpet and that nasty smell.