I have a Intertherm central heat & air unit model number asco36bcraa and product date code 83JY-21-01 my problem is the blower will only run in the on position of the themostat when I put it to auto it does not run.the heating elements and the thermostat work as they should.so my question is why don't the blower work in auto on the thermostat?

Because the auto switch has frozen in the open position not allowing it to function properly. It needs replaced in order to have your furnace run properly. Any repairdude can do the repair in less than 1/2 hour once he has the right part.