1993 Suburban heater control

Our front heater control went offline completly, we have check fuses and replaced fuses and now have replaced the complete heater control unit with no effect. the other thing is that when the headlights ore off the buttons on the control are lit, when the lights are turned on the button lights go off. With the head light on and you dim the dash lights to off the buttons on the heater control light up but with the dash lights on the heater control is dark.vAny ideas where I might have a short?