Blower motor only works at high speed

Blower motor only works at high speed. Original blower motor began to work only if I went out and tapped it with a hammer. Replaced it, then new motor only worked at high speed. Replaced Resistor. No real change. Replaced fuses, fan switch, 50 amp fuse in Power Distribution Box under hood. No change. What gives? What do I do next? The only other variables I can even think of are either (1) a fugitive grounding problem somewhere or (2) the plug from the wiring harness may be bad (but does not look to be).

Help?

Thanks !