TJ heater blower only works on High? Here's why
There is a resistor pack on the blower motor that slows the speed from High (the default, and only un-restricted speed that bypasses the resistor). This resistor pack slows the fan in the first few positions. It is located under the passenger side of the dash, under (well, above, technically) the black kick panel, next to the right side wall. It's only a few screws and an electrical connector holding it in. The part costs about $22, and takes less than 10 minutes to install.