The heater resistor pack is bad. Replace it. A Blower Motor Resistor burned down and needs to be replaced. It costs about $37 from the dealer and is located on the passenger side underneath the dashboard. It is held there by 2 screws and very easy to change. Common problem with Nissan cars.
Blower motor resistor...around $25 on eBay
Look under the dash on the passenger side
The problem might be with the switch itself. The contacts on the switch may be dirty, or the switch is worn out, causing the fan to stay on.
Why are you replacing the switch? If the fan works only on high speed, replace the fan resistor. There was a TSB for this.
There is a resistor near the blower that is bad, known cause is high speed only.
Could be a resistor problem.
Be more specific, if the fan speed only works on high then it is most cases a bad resistor board for the fan, a $10-20 part, available from an advance, murrays, or lee or similar auto parts store.
If you mean the fan works on high only it's the blower motor resistor.
It could be the blower fan resistor or the relay
YOUR BLOWER RESISTER IS BAD
either the switch or the fan motor
That would be caused by the blower resistor.