If the blower resistor fails you get hi fan or nothing at all. Hi speed on the switch bypasses the resistor but some setups have a fuse built into the resistor assembly.
So your saying the fan only runs in the low speed switch position? Or does it run slow in all switch positions?
If it's only running in low position and you've replaced the resistor, I would say it's the switch.
If it runs slow in all positions, I would look for a bad connection like a bad ground or a connector in the system.