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Sounds like a couple of issues. One, your blower fan could be going bad, the resistor pack could be going bad, and/or your control switch could be failing.
Second, a blend door or actuator could be stuck.
Unless you're really handy, I recommend taking it to a qualified mechanic to have these two items looked at.
Turn the blower fan on high and see if the heat goes away.
It may be the low speed fan resistor, sometimes it's directly behind the control.
Did you try other settings? Heat instead of AC, AC regular vs AC MAX, or just Vent?
You didn't tell us what vehicle or year and we're not the psychic hotline, so this is my best guess.
This is a diagram of a blower relay and a fan motor, the normally closed fan relay switch goes to the low speed which is the Heat speed, the low speed is controllled by a temperature delay depicted in the low speed circuit.
The normally open fan relay switch goes to the high speed, which is the AC modes speed , also if you run the fan continuously via the fan switch on the thermostat this is also to high speed.
I suspect the temperature delay's on setting is being reached thus cycling the low speed of the fan.
If you have a fan that has different speed settings (1234, or low med high etc) and the only one that works is high - then the blower resistor pack has failed. If you have a climate control unit (where you just pick a temp you want) then the fan is controlled by the heating system and it sets fan speed. If you have climate control style try lowering the temp to a lower setting (current temp in vehicle) and see if fan slows down. If not then climate control unit or fan resistor bad. If dial type and you get nothing on lower settings (only high works like mentioned above) you need to change blower resistor. It is located under dash near the fan itself. Look for small unit bolted in duct near fan with wire plug going to it.
On any other vehicle it sounds like the blower fan switch. The fan switch provides power or ground to the resistor and there would be a high speed blower relay to bypass the resistor. But in your case the fan switch is part of the controller. I don't have a diagram for a 92 but the 94 model shows relays for low medium and high speeds. Each relay gets power from a fuseable link. The high relay gets power thru the low relay if the low relay is not engaged. If you have the model with 3 relays you may need to check them.
So your saying that the blower motor will not blow anything hot or cold.
You probably need a new blower motor . Its not likely that all speeds would go especially high speed since all the other speeds go threw a resistor and high speed is direct B+ from the battery. Start therebefore I have you testing a/c parts. Keep me posted.
It's possible you are not getting enough coolant flow to your heater core. Try pulling the hoses off the core, blowing it out with air compressor, fill with water and blow out again. Try this about five times and see what happens. Alot of times these cores will plug up or have restricted flow. They will blow warm at low speed but it won't flow enough hot coolant through to stay hot on high blower speed.
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