1999 Cadillac Deville heater does not get hot

Cold on both sides? If you feel the heater hoses where they enter the core(behind rh strut tower), are they hot? If so you have an actuator or blend door problem. If you can place your hand on all hoses after the vehicle has reached what is perceived to be operating temp, you could have a thermostat that is stuck open, or not regulating temperature. By using primarily 'auto', you would also have refrigerant flowing in the evaporator, which would cool down the warm air even more. Also verify the coolant is full.