My car's fan switch, the one that selects the 4 speeds, does not work properly, the position that works is when I select position number 4, which is the highest speed.
This was some time ago, but last week I noticed that this switch does not start the A/C as it used to do !!!!!!! (even on 4)

 

You have two seperate issues here.

1) The fan speed - Change the fan resistor. It's located under the dash, passenger side, on the HVAC duct work after the blower motor. There's a plug with a few wires going to it, thats the resistor block. Get a new one from Nissan first so you know what your looking for when your down under there.

2) Your A/C system either lost some R134 or the temp is below 30F. Either of the two will cut the circuit so the compressor clutch woun't engauge.