1993 Cadillac Deville

The heater control valve is at the center rear of engine compartment, in line with the heater hoses. Check first that it gets a vacuum signal, and the line is connected, then check if it is hot on both sides. You can also move the lever that opens the valve. If this corrects it, and it is getting a vacuum signal, it needs a valve. If the hoses are hot through the hot water valve, check at the core. If it is hot on one hose and not the other, it could have a restricted heater core. This all assumes that the basics have been checked, and the coolant is full.