Cold on low and cool on high suggests a capacity problem and not a problem with the heater circuit.

Check for dirt in the condenser and radiator, and that your fan shroud is properly attached. Also you may have a dirty evaporator preventing flow there. The old models did not use a filter in the cabin air path, so the evaporator collects the dirt. If the outside mechanicals are fine, extract and weigh the refrigerant to determine if you have the correct quantity.