My heater A/C control works only in the high positon or the 4th position.

You need to replace the "blower motor resistor" pack. The blower motor has 12v constantly from the HVAC relay. The high position bypasses the resistor pack and give the blower straight ground from the switch. The other 3 positions (Low, M1 and M2) give the blower ground through the resistor pack, thus slowing the motor down from full speed.

99% of the time it is a failed blower resister.