My mother's 2002 ford taurus has no heat. When

My mother's 2002 ford taurus has no heat. When she turns the knob to heat, the air conditioner comes on. You can hear the air conditioner compresser, it keeps kicking in. The air conditioner works fine. Mom's brother in-law, unpluged the air conditioner and warm air is blowing out. But when he plugs the air conditioner back up, it blows cold air, with the compresser kicking in. Does anyone know what maybe causing this or has anyone had this same problem with the ford taurus and if so, what it took to correct the problem. thanks