I turn the a/c on but it doesn't blow air.

Try rapping on the motor when it won't go on. The brushes may be worn out and don't make contact like they used to. If that doesn't help, check the wiring and connectors between the switch, the resistor, and motor. It's possible that the switch is bad but I'd think about the motor first. It's a very simple system and should be easy to diagnose. If you want to find a specialist, here's our directory link for you: https://repairpal.com/nissan-repair-in-miami-florida