Sounds like a wiring issue, does it work ok at full setting? If so it could be the resistors which slow the motor have packed in, loose/corroded wires or the switch is at fault.
i had a similar problem with fan speed and hot/cold. the cables were slipping. a simple adjustment to tighten cables. try moving the adjustment on the actual heater box, not on dash.
My heater started to work again , all of its own accord.
Great , I thought, then it packed up again
It seems to stop working after its warmed the inside up to bikini hot temperatures. Its did this Tuesday morning on way to school then packed up on way to work but fortunately worked again today.....brrr..snow