It can be a fuse or the switch itself or the resistor pack.
The 1999 Nissan Sentra has a blower resistor in the air vents. This mechanism regulates the amount of air entering the cabin.
The Nissan blower motor can be found in the engine compartment on the firewall. The blower motor will be on the passenger side of the firewall.
They are generally found close to the blower motor.
to the right of the gas peddal
Defective blower motor resistor pack.
A bad control switch can cause your 2003 Nissan Sentra heater blower to stay on. The motor or a bad sensor can also cause the heather not to work properly.
I had the same problem and I found it to be the blower motor resister. This is what controls the blower motor settings. You will have to find out where it is on yours.
More than likely the blower motor speed resistor burned out.
Blower resistor switch
you need a new blower motor resistor
Your blower motor is on the passenger side of the engine bay mounted on the firewall. It should be just a couple of screws and a wire connector to deal with.
Locate the resistor under the dash attached on the blower motor housing. Disconnect the wire harness connector. Remove the resistor and install a new one.