If the heater and A/C fan only blows on the highest setting and goes dead on low and medium, the blower resistor pack has burned open. It is the number-one cause of missing fan speeds on any Miata. High works because that speed bypasses the resistor and runs the motor directly.
The resistor pack sits in the HVAC ducting on the passenger side, in the airflow so it stays cool, and it drops voltage to give you the lower speeds
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