1991 ford mustang heater?
The first thing to look at is the thermostat. Does the car have a temperature gauge? If so, you may see the engine temperature staying low, well below the middle of the gauge. That would indicate a thermostat that is not closing properly, which would cause you to have insufficient heat. If there is not temperature gauge, you could ask you mechanic to check the coolant temperature by simply placing a water heater thermometer into the radiator opening. The temperature should be about 190. If the thermostat is not closing properly, it will be as low as 150 or 140. Having the thermostat replaced is not difficult, and should not be expensive.