95 mustang air conditioning

my air conditioner smells like poo when I turn it on. The heater does not have an odor. What causes this and can it be corrected?

Two possibilities I can think of: either there’s a rodent living in the ducting, or the evaporator condensation drain is blocked up. When the A/C has been running, there should be a steady drip of water onto the ground. If there isn’t, the drain is your problem and you might have mold/mildew buildup as a result.