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

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.