My fan blower works on speeds 1/2 but not 3/4 and my a/c is not working. any thoughts?

Welcome to NICO!

Bummer that you're having trouble. In GM cars, this is usually resolved with a new resistor, but I'm not positive about the Nissan side of things.

However, we may still be able to help you out. Here at NICO, we have Factory Service Manuals available free to members. Take a peek at the link in my signature and find the FSM for your car and you'll have all the info you'll ever need to make the repair and try to diagnose it for yourself while help is on the way.
Take a peek at the A/C section and look for a diagnostic tree to help you start finding some solutions.