In most cases the blower motor resistor is the solution. But you need to have a repair shop check it out before you buy something that you may not need.
I had a similar problem when one of the speeds on my fan control stopped working. I had to replace the blower motor resistor. It was about $10 at the local auto shop and took about ~45mins to get it in. You could also look at the blower motor it could be blown.
Why does your ac and heater fan turn off and on
blower fan location on a 1979 lincoln
The fan blower speed resistor is burned out
The ac blower fan is removed with three screws from the under side of the dash on the passenger side.The ac blower fan is removed with three screws from the under side of the dash on the passenger side.
Your AC fan is not working on the maximum setting because you have a bad blower motor resistor. This is located under the dash or next to the blower motor.
I would look into the blower motor resistor module
You did not mention the kind of car. Check the fuses and relay to be sure you have power. Also you have a blower motor resistor and the fan motor. Grounds are just as important as the power source.
The blower fans on these cars run very quietly on all but the maximum setting- ensure that the fan is really not working first by holding your hand in front of the vent with the fan on low and checking that is it not moving any air. Replace the resistor pack.
I'm having problems with the ac blower of my Tahoe in front.
Check for a blown fuse first. If the fuse is OK, then the blower motor resistor pack or the blower motor itself may be bad.
Try checking the blower motor fuse, it may be blown. Also, check the wiring harness for good contact to motor connection.Last, replace the blower motor , ~$30 and 20 minutes to replace it, watch for AC lines when working near them. Good luck!!