Sounds like multiple issues here. With regard to the fan speed only working on high, it is probably a bad blower resistor pack. With the weak heat, this could be caused by several things. My first concern is that there is not enough coolant in the system, which would not keep coolant flowing consistently through the heater core, thereby leaving not very hot air from the vent. This is a dangerous condition as you can potentially overheat and damage your water pump and/or engine.