One problem related to defogger controls has been reported for the 1998 Chevrolet Astro. The most recently reported issues are listed below.
Switch on dash board for rear air conditioner fan control was over heating in the high speed position. I tried to turn it off and could not and it was hot to the touch. I knew how to remove the switch while my wife was driving. The switch was over heating and partially melted the connector and destroyed the switch. I believe that if I did not disconnect the switch the possibility of a fire was there.