Why does A/C and Heater blower work only on high speed?

For serveral months blower worked only on high speed. Recently blower quit working altogether. Found that connector to blower motor was melted. Replaced pigtail connector to blower motor and blower motor in case it had to much resistance that might have caused connector to overheat and melt. Also, replaced resistor at this time. Blower motor working again, but only on high speed. Then, I replaced manual speed select switch in the control panel and the blower relay on firewall. These changes did not help, still only high speed. In summary, fuse #7 in fuse panel good, replaced blower motor, connector to blower motor, resistor, switch and relay. Can you please tell me what else to check, or detail instructions on how to troubleshoot this problem.