replace the blower motor resistor located on the passenger side under dash up in the corner.
you need a new rheostat !
A common problem is that the heater resistor/ circuit board is defective and need to be replaced. The main symptom is that the heater fan with not run on the three low speeds but will run on high.
could be thermostat or heater fan if you can hear all the speeds on the blower motor working then it could be your thermostat but your heater core might be plugged as well
If it has only high speed, the blower motor resistor has to be replaced to get all speeds back.
Sounds more like switch or resistor is defective
I had one of these on a 1991 Wrangler and if I remember correctly it had 3 speeds. i currently have a 92 and it dose have 3 speeds
Either than switch, the resistor, or the relay is bad. On my 1999 Buick Century, the blower motor quit working on the three lower speeds, and then went out completely. When the resistor goes bad, the lower speeds quit working. My blower motor was squealing, also. After I replaced the blower motor and the resistor, I tested it and found the blower worked great on every speed but high. It turned out that there is a separate circuit for high speed, and the fuse was blown. I replaced the fuse for the high speed, and the blower now works on all 5 speeds. Replacing the blower and resistor was difficult, but not as bad as expected, thanks to help from this site.
Heater blower resistor is what controls the low medium high speeds...
The blower motor resistor is bad.
It sounds like the blower resistor assembly is defective. All fan speeds except for high go through resistors (that is what controls the speeds from low to high). Number 4 is high and has the full 12 volts going to the fan.
There's a sensor in the duct work behind th glove box under the dash that controls the fan speeds. It could be that. On my truck I only had the two highest speeds and that's the part I changed.
I you're talking about blower speeds 1 and 2 not working, then it's the resistor. Instructions on how to replace it are here: http://www.automotiveforums.com/vbulletin/showthread.php?t=175159