I have a ’99 Taurus that when I first got it a year ago would blow warm air but very faintly and then not at all. I added some coolant and it bagan to blow warm air for a little while if I turned it to max AC. The AC also wouldn’t blow air that was very cold but before I could get it looked at this spring it had a day that it suddenly started working full blast during a road trip and then when I went to turn the car on the next day the whole system (fan and AC) were dead. It makes the psst sound when I turn the knob but nothing beyond that. I haven’t had the money to get it taken care of since I assumed it was a major issue, but then last week when I was driving my son to the bus stop it suddenly kicked on and worked perfectly other than still no warm air. Then when I started it up to go to work the system was dead again. I am wondering if maybe it could be a short or something. This winter when we had a week of very frigid temperatures it would run fine while I let the car run to defrost but then once I tried to shift into gear or shortly after the car would die and I would have to have it jumped. Once it was jumped it was fine until the next morning when the same thing would happen. I took it to a mechanic and they were unable to find any issue at the time, although they were looking more for an issue with the electrical system.
It’s a definate short as far as the blower. The heat issue is seperate, probably a semiplugged. Usually a good flush and cleaning will fix the heater core. Look for electrical connectors that have been seperated or an aftermarket radio installation where wire have been cut or spliced or possibly a bad fan resistor or switch.