I am going crazy I have a 96 Trans-Am and the heater selector switch, between panel-defrost-floor settings doesn't work anymore. I am pulling my hair out trying to fix this. I have already replaced the control unit, thinking it may be a vaccum problem. Anyone with any info would be a great help.

 

If it is stuck on defrost, then you have a vacuum problem and need to find out where the line either got kinked or melted.

 

yea...if its stuck on defrost check the main vacuum supply line. it will default into defrost as a safety precaution.