On my 1989 GMC S15 (4 cyl) the heater core needs replaced due to coolant leaking

Hello, the heater core is a separate component from your air conditioning system. However, your heater blower is the same blower fan that blows the cold air when the blend door in the plenum (the box behind the dash that contains both the heater core AND the air conditioning evaporator core...one produces heat and the other cold to blow air through). If your blower motor is producing inefficient air from the heat side, it would do the same for the cold side also. If coolant is present in the plenum area due to the heater core leak also, it could present further challenges. Does this help? Good luck!