What is wrong if you replace the heater switch but the heater continues to only blow on the 3rd click and nothing else?
The relay is bad.
Why will the heater in a 1996 Grand Am with 4 pos heater switch not work on HIGH but it works in all 3 other positions?
If you are talking about a wall switch for the heater, check to see if the heater has its own internal thermostat or switch as this might be in the off position and not allow the heater to turn on. If you are talking about a switch on the heater itself, then yes in all probability the switch is not working.