the a/c vent fan always works on high, and sometimes on low, never in between

It should be your blower motor relay. It's an easy fix, and cheap. I have done a few in my past cars without an problem. I got my parts at auto zone.

you mean the blower motor fan speeds work on high or low.pull the blower motor resistor out ,it's burnt,replace it.it's not the relay!pull the resistor out and look at it on the end.the part number is JA1772 at the ZONE,and is $52.99 plus tax.google that part number,see what it looks like.