Both the air or the heat do not switch on at all. What could be the cause?

I just went thru this. My vents he nothing coming out. Fist I replaced resistor resistor usually bad if it starts out where it only works on 4 and not 1-3 well that wasn't my problem so I next changed my relay and that all did nothing so I took blower motor out hooked to a battery an it works with a push start I put back in after cleaning and oilin really good. Worked for 3 days and stopped so I bought new one well the wiring on replacement was backwords I couldn't figure out y my vents was cold bit nothing as air pressure come to find out I had to take the plug on new blower motor an swap the positive and negative cable as they was backwords from originally one. So try blower motor and make sure your wiring is the same place in new one as old as far as your posive and negative. My quest is 99 gxe with 142,000 Miles hope.my experience helps you as I was guessing and not a mechanic just a desperate mom who knows her way around tools and cars super handy.