I just replaced the heater controls on the dash. The new control unit does not show that the a/c is on while using defrost setting, although I can hear that the compressor is on. Before I changed the unit, the a/c light would come on and if I pushed the a/c button the light would go off and the compressor would stop. I no longer have this option. Is there a trick to programming the unit so it operates like it used to?