I've seen some wiring diagrams for changing over these pos thermostats to digital ones...I picked up a digital thermostat and wired it, but couldn't get just fan to work, nor the high fan ? No, I didn't wire a separate switch to the high side...I have no need for the low side, so I wired the high direct.
The low fan is necessary if it's not too hot outside, but humid. The high fan in moderate temps will cause the unit to short cycle and it will not remove enough humidity before it knocks off. The low fan will let the unit run longer before the set temp is reached and in doing so will remove more moisture from your rig. Some of the newer thermostats have "auto" fan which chooses high or low for you depending on how far above the set temp it is.