I when you turn the switch on do you get current where the harness plugs into the motor? Years ago, in the early 80’s, my dad had a blower motor that would work when connected to 12 volt power supply but not when connected to the car. Got a new blower motor and it worked fine. Also, I have seen my fair share of bad out of box aftermarket fan relays.
Check for current where the harness plugs into fan.