Start with the black wire at the blower motor itself. That's your power to the blower. If you have a voltmeter, look for full battery voltage there. *The blower motor does have a control relay so the HVAC needs to be on in some capacity.
Next, I'd just do a visual inspection of the connector at the resistor. Look for burnt or shorted wires. Anything showing signs of being melted should be replaced.
Can I assume that the resistor itself has been replaced already and do you have the basic four speed switch or the infinitely variable blower switch on your HVAC panel?
When I switched out the resistor, twice, I checked the wires and took the heater control out and checked that too. I thought maybe the switch was bad but no sign of that. The vehicle had a wire cage resistor and the only one I could get was like a lollipop and didn't last long. I found a cage one and put it in.
The motor itself could be failing then. They're a nightmare to remove just because of one screw that's tucked up really close to the firewall. I'm a big ol fat guy so I was never ashamed to add the extra step of removing the seat so I could work with some kind of comfort. It might be worth the misery of one screw to find out if you have a bad motor.