How is it failing? leak, plugged etc.? There is a technical service bullitin regarding this : # 01-15-6. Essentially it has to do with most Ford products. There may be a ground problem, or lack of coolant flushes. You can get the complete service bulletin at this address: http://www.alldata.com/recalls/
A Ford Mustang does not have a heater coil. You must be thinking of your home HVAC.
No...
its got toothpaste in the carby
No
Check the resistor pack on the evaporator case under the hood
It rusted out
alternator
a bad water pump could cause low heat. The heater uses the coolant from the pump to generate heat.
windshield leaking heater core or heater core hoses.
I would check all the hoses if they are all tight I would go ahead and replace it due to age.
There are many reasons why the anti-lock light would turn on in a 1994 Ford Mustang GT. One reason are bad ABS sensors.
Your heater relay switch may be bad